THE 


MICROSCOPE. 

BEING  THE  ABTICLE  CONTKIBUTED  BY 

ANDREW     ROSS 

TO    THE    " PENNY    CYCLOPEDIA,"    PUBLISHED    BY  THE   SOCIETY 
FOB  THE  DIFFUSION   OF  USEFUL  KNOWLEDGE. 


'_£ 


'7  T 


FULLY     I  LLUSTRATED. 


NEW  YOKE: 
TPIE    INDUSTBIAL    PUBLICATION    COMPANY. 

1877. 


THE  MICROSCOPE. 


ICKOSCOPE,  the  name  of  an  in- 
strument for  enabling  the  eye  to 
see  distinctly  objects  which  are 
placed  at  a  very  short  distance 
from  it,  or  to  see  magnified  images 
of  small  objects,  and  therefore  to 
see  smaller  objects  than  would 
otherwise  be  visible.  The  name 
is  derived  from  the  two  Greek 
words,  expressing  this  property, 
MIKBOS,  small,  and  SKOPEO,  to  see. 

So  little  is  known  of  the  early 
history  of  the  microscope,  and  so 
certain  is  it  that  the  magnifying 
power  of  lenses  must  have  been 
discovered  as  soon  as  lenses  were  made,  that  there  is  no  reason 
for  hazarding  any  doubtful  speculations  on  the  question  of 
discovery.  We  shall  proceed  therefore  at  once  to  describe  the 
simplest  forms  of  microscopes,  to  explain  their  later  and  more 
important  improvements,  and  finally  to  exhibit  the  instrument 
in  its  present  perfect  state. 

In  doing  this  we  shall  assume  that  the  reader  is  familiar  with 
the  information  contained  in  the  articles  "  Light/'  "Lens," 
"Achromatic,"  "Aberration,"  and  the  other  sub-divisions  of 
the  science  of  Optics,  which  are  treated  of  in  this  work. 

The  use  of  the  term  magnifying  has  led  many  into  a  miscon- 
ception of  the  nature  of  the  effect  produced  by  convex  lenses. 
It  is  not  always  understood  that  the  so-called  magnifying  power 
of  a  lens  applied  to  the  eye,  as  in  a  microscope,  is  derived  from 


4  THE    MICKOSCOPE. 

its  enabling  the  eye  to  approach  more  nearly  to  its  object  than 
would  otherwise  be  compatible  with  distinct  vision.  The  com- 
mon occurrence  of  walking  across  the  street  to  read  a  bill  is  in 
fact  magnifying  the  bill  by  approach;  and  the  observer,  at 
every  step  he  takes,  makes  a  change  in  the  optical  arrangement 
of  his  eye,  to  adapt  it  to  the  lessening  distance  between  him- 
self and  the  object  of  his  inquiry.  This  power  of  spontaneous 
adjustment  is  so  unconsciously  exerted,  that  unless  the  atten- 
tion be  called  to  it  by  circumstances,  we  are  totally  unaware  of 
its  exercise. 

In  the  case  just  mentioned  the  bill  would  be  read  with  eyes 
in  a  very  different  state  of  adjustment  from  that  in  which  it 
was  discovered  on  the  opposite  side  of  the  street,  but  no  con- 
viction of  this  fact  would  be  impressed  upon  the  mind.  If, 
however,  the  supposed  individual  should  perceive  on  some  part 
of  the  paper  a  small  speck,  which  he  suspects  to  be  a  minute 
insect,  and  if  he  should  attempt  a  very  close  approach  of  his 
eye  for  the  purpose  of  verifying  his  suspicion,  he  would  pre- 
sently find  that  the  power  of  natural  adjustment  has  a  limit; 
for  when  his  eye  has  arrived  within  about  ten  inches,  he  will 
discover  that  a  further  approach  produces  only  confusion.  But 
if,  as  he  continues  to  approach,  he  were  to  place  before  his  eye 
a  series  of  properly  arranged  convex  lenses,  he  would  see  the 
object  gradually  and  distinctly  increase  in  apparent  size  by  the 
mere  continuance  of  the  operation  of  approaching.  Yet  the 
glasses  applied  to  the  eye  during  the  approach  from  ten  inches 
to  one  inch,  would  have  done  nothing  more  t&an  had  been  pre- 
viously done  by  the  eye  itself  during  the  approach  from  fifty 
feet  to  one  foot.  In  both  cases  the  magnifying  is  effected 
really  by  the  approach,  the  lenses  merely  rendering  the  latter 
periods  of  the  approach  compatible  with  distinct  vision. 

A  very  striking  proof  of  this  statement  may  be  obtained  by 
the  following  simple  and  instructive  experiment.  Take  any 
minute  object,  a  very  small  insect  for  instance,  held  on  a  pin  or 
gummed  to  a  slip  of  glass;  then  present  it  to  a  strong  light, 
and  look  at  it  through  the  finest  needle-hole  in  a  blackened 
card  placed  about  an  inch  before  it.  The  insect  will  appear 
quite  distinct,  and  about  ten  times  larger  than  its  usual  size. 
Then  suddenly  withdraw  the  card  without  disturbing  the  ob- 


THE    MICKOSCOPE.  5 

ject,  which  will  instantly  become  indistinct  and  nearly  invisi- 
ble. The  reason  is,  that  the  naked  eye  cannot  see  at  so  small  a 
distance  as  one  inch.  But  the  card  with  the  hole  having  en- 
abled the  eye  to  approach  within  an  inch,  and  to  see  distinctly 
at  that  distance,  is  thus  proved  to  be  as  decidedly  a  magnifying 
instrument  as  any  lens  or  combination  of  lenses. 

This  description  of  magnifying  power  does  not  apply  to  such 
instruments  as  the  solar  or  gas  microscope,  by  which  we  look 
not  at  the  object  itself,  but  at  its  shadow  or  picture  on  the  wall; 
and  the  description  will  require  some  modification  in  treating 
of  the  compound  microscope,  where,  as  in  the  telescope,  an 
image  or  picture  is  formed  by  one  lens,  that  image  or  picture 
being  viewed  as  an  original  object  by  another  lens. 

It  is  nevertheless  so  important  to  obtain  a  clear  notion  of  the 
real  nature  of  the  effect  produced  by  a  lens  applied  to  the  eye, 
that  we  will  adduce  the  instance  of  spectacles  to  render  the 
point  more  familiar.  If  the  person  who  has  been  supposed  to 
cross  the  street  for  the  purpose  of  reading  a  bill  had  been  aged, 
the  limit  to  the  power  of  adjustment  would  have  been  discov- 
ered at  a  greater  distance,  and  without  so  severe  a  test  as  the 
supposed  insect.  The  eyes  of  the  very  aged  generally  lose  the 
power  of  adjustment  at  a  distance  of  thirty  or  forty  inches 
instead  of  ten,  and  the  spectacles  worn  in  consequence  are  as 
much  magnifying  glasses  to  them  as  the  lenses  employed  by 
younger  eyes  to  examine  the  most  minute  objects.  Spectacles 
are  magnifying  glasses  to  the  aged  because  they  enable  such 
persons  to  see  as  closely  to  their  objects  as  the  young,  and 
therefore  to  see  the  objects  larger  than  they  could  themselves 
otherwise  see  them,  but  not  larger  than  they  are  seen  by  the 
unassisted  younger  eye. 

In  saying  that  an  object  appears  larger  at  one  time,  or  to  one 
person,  than  another,  it  is  necessary  to  guard  against  miscon- 
ception. By  the  apparent  size  of  an  object  we  mean  the  angle 
it  subtends  at  the  eye,  or  the  angle  formed  by  two  lines  drawn 
from  the  centre  of  the  eye  to  the  extremities  of  the  object. 
In  Fig.  1,  the  lines  A  E  and  B  E  drawn  from  the  arrow  to  the 
eye  form  the  angle  A  E  B,  which,  when  the  angle  is  small,  is 
nearly  twice  as  great  as  the  angle  C  E  D,  formed  by  lines  drawn 
from  a  similar  arrow  at  twice  the  distance.  The  arrow  A  B 


THE    MICROSCOPE. 


B 
Fig.  1. 


will  therefore  appear  nearly  twice  as  long  as  C  D,  being  seen 
under  twice  the  angle,  and  in  the  same  proportion  for  any 

greater  or  lesser  difference 
in  distance.  The  angle  in 
question  is  called  the  angle 
of  vision,  or  the  visual  an- 
gle. 

The  angle  of  vision  must, 
however,  not  be  confounded 
with  the  angle  of  the  pencil 
of  light  by  which  an  object 

is  seen,  and  which  is  explained  in  Fig.  2.  Here  we  have  drawn 
two  arrows  placed  in  relation  to  the  eye  as  before,  and  from 
the  centre  of  each  have  drawn  lines  exhibiting  the  quantity 
of  light  which  each  point  will  send  into  the  eye  at  the  respec- 
tive distances. 

Now  if  E  F  represent  the  diameter  of  the  pupil,  the  angle 
E  A  F  shows  the  size  of  the  cone  or  pencil  of  light  which  enters 
the  eye  from  the  point  A,  and  in  like  manner  the  angle  E  B  F 
is  that  of  the  pencil  emanating 
from  B,  and  entering  the  eye. 
Then,  since  E  A  F  is  double  E 
B  F,  it  is  evident  that  A  is  seen 
by  four  times  the  quantity  of 
light  which  could  be  received 
from  an  equally  illuminated 
point  at  B;  so  that  the  nearer 
body  would  appear  brighter  if 

it  did  not  appear  larger;  but  as  its  apparent  area  is  increased 
four  times  as  well  as  its  light,  no  difference  in  this  respect  is 
discovered.  But  if  we  could  find  means  to  send  into  the  eye  a 
larger  pencil  of  light,  as  for  instance  that  shown  by  the  lines 
G  A  H,  without  increasing  the  apparent  size  in  the  same  pro- 
portion, it  is  evident  that  we  should  obtain  a  benefit  totally 
distinct  from  that  of  increased  magnitude,  and  one  which  is  in 
some  cases  of  even  more  importance  than  size  in  developing  the 
structure  of  what  we  wish  to  examine.  This,  it  will  be  here- 
after shown,  is  sometimes  done;  for  the  present,  we  wish  merely 
to  explain  clearly  the  distinction  between  apparent  magnitude, 


Fig.  2. 


rnr,   MICROSCOPE.  7 

or  the  angle  under  which  the  object  is  seen,  and  apparent 
brightness,  or  the  angle  of  the  pencil  of  light  by  which  each  of 
its  points  is  seen,  and  with  these  explanations  we  shall  continue 
to  employ  the  common  expressions  magnifying  glass  and  mag- 
nifying power. 

The  magnifying  power  of  a  single  lens  depends  upon  its  focal 
length,  the  object  being  in  fact  placed  nearly  in  its  principal 
focus,  or  so  that  the  light  which  diverges  from  each  point  may, 
after  refraction  by  the  lens,  proceed  in  parallel  lines  to  the  eye, 
or  as  nearly  so  as  is  requisite  for  distinct  vision.  In  Fig.  3, 
A  B  is  a  double  convex  lens,  near  which  is  a  small  arrow  to 


D. 

Fig.  3. 

represent  the  object  under  examination,  and  the  cones  drawn 
from  its  extremities  are  portions  of  the  rays  of  light  diverging 
from  those  points  and  falling  upon  the  lens.  These  rays,  if 
suffered  to  fall  at  once  upon  the  pupil,  would  be  too  divergent 
to  permit  their  being  brought  to  a  focus  upon  the  retina  by 
the  optical  arrangements  of  the  eye.  But  being  first  passed 
through  the  lens,  they  are  bent  into  nearly  parallel  lines,  or 
into  lines  diverging  from  some  points  within  the  limits  of  dis- 
tinct vision,  as  from  C  and  D.  Thus  altered,  the  eye  receives 
them  precisely  as  if  they  emanated  from  a  larger  arrow  placed 
at  0  D,  which  we  may  suppose  to  be  ten  inches  from  the  eye, 
and  then  the  difference  between  the  real  and  the  imaginary 
arrow  is  called  the  magnifying  power  of  the  lens  in  question. 

From  what  has  been  said  it  will  be  evident  that  two  persons 
whose  eyes  differed  as  to  the  distance  at  which  they  obtained 


THE    MICROSCOPE. 

distinct  vision,  would  give  different  results  as  to  the  magnify- 
ing power  of  a  lens.  To  one  who  can  see  distinctly  with  the 
naked  eye  at  a  distance  of  five  inches,  the  magnifying  power 
would  seem  and  would  indeed  be  only  half  what  we  have 
assumed.  Such  instances  are,  however,  rare;  the  focal  length 
of  the  eye  usually  ranges  from  six  to  twelve  or  fourteen  inches, 
so  that  the  distance  we  first  assumed  of  ten  inches  is  very  near 
the  true  average,  and  is  a  convenient  number,  inasmuch  as  a 
cipher  added  to  the  denominator  of  the  fraction  which  expresses 
the  focal  length  of  a  lens  gives  its  magnifying  power.  Thus  a 
lens  whose  focal  length  is  one-sixteenth  of  an  inch  is  said  to 
magnify  160  times. 

When  the  focal  length  of  a  lens  is  very  small,  it  is  difficult  to 
measure  accurately  the  distance  between  its  centre  and  its 
object.  In  such  cases  the  best  way  to  obtain  the  focal  length 
for  parallel  or  nearly  parallel  rays  is  to  view  the  image  of  some 
distant  object  formed  by  the  lens  in  question  through  another 
lens  of  one  inch  solar  focal  length,  keeping  both  eyes  open  and 
comparing  the  image  presented  through  the  two  lenses  with 
that  of  the  naked  eye.  The  proportion  between  the  two 
images  so  seen  will  be  the  focal  length  required.  Thus  if  the 
image  seen  by  the  naked  eye  is  ten  times  as  large  as  that  shown 
by  the  lenses,  the  focal  length  of  the  lens  in  question  is  one- 
tenth  of  an  inch.  The  panes  of  glass  in  a  window,  or  courses 
of  bricks  in  a  wall,  are  convenient  objects  for  this  purpose. 

In  whichever  way  the  focal  length  of  the  lens  is  ascertained, 
the  rules  given  for  deducing  its  magnifying  power  are  not 
rigorously  correct,  though  they  are  sufficiently  so  for  all  prac- 
tical purposes,  particularly  as  the  whole  rests  on  an  assumption 
in  regard  to  the  focal  length  of  the  eye,  and  as  it  does  not  in 
any  way  affect  the  actual  measurement  of  the  object.  To  cal- 
culate with  great  precision  the  magnifying  power  of  a  lens  with 
a  given  focal  length  of  eye,  it  is  necessary  that  the  thickness  of 
the  lens  be  taken  into  the  account,  and  also  the  focal  length  of 
the  eye  itself. 

We  have  hitherto  considered  a  magnifying  lens  only  in  refer- 
ence to  its  enlargement  of  the  object,  or  the  increase  of  the 
angle  under  which  the  object  is  seen.  A  further  and  equally 
important  consideration  is  that  of  the  number  of  rays  or  quan- 


THE    MIOKOSCOPE.  9 

tity  of  light  by  which  every  point  of  the  object  is  rendered 
visible.  The  naked  eye,  as  shown  in  Fig.  2,  admits  from  each 
point  of  every  visible  object  a  cone  of  light  having  the  di- 
ameter of  the  pupil  for  its  base,  and  most  persons  are  familiar 
with  that  beautiful  provision  by  which  in  cases  of  excessive 
brilliancy  the  pupil  spontaneously  contracts  to  reduce  the  cone 
of  admitted  light  within  bearable  limits.  This  effect  is  still 
further  produced  in  the  experiment  already  described,  of  look- 
ing at  an  object  through  a  needle-hole  in  a  card,  which  is 
equivalent  to  reducing  the  pupil  to  the  size  of  a  needle-hole. 
Seen  in  this  way  the  object  becomes  comparatively  dark  or  ob- 
scure; because  each  point  is  seen  by  means  of  a  very  small 
cone  of  light,  and  a  little  consideration  will  suffice  to  explain 
the  different  effects  produced  by  the  needle-hole  and  the  lens. 
Both  change  the  angular  value  of  the  cone  of  light  presented 
to  the  eye,  but  the  lens  changes  the  angle  by  bending  the  ex- 
treme rays  within  the  limits  suited  to  distinct  vision,  while  the 
needle-hole  effects  the  same  purpose  by  cutting  off  the  rays 
which  exceed  those  limits. 

It  has  been  shown  that  removing  a  brilliant  object  to  a 
greater  distance  will  reduce  the  quantity  of  light  which  each 
point  sends  into  the  eye,  as  effectually  as  viewing  it  through  a 
needle-hole;  and  magnifying  an  object  by  a  lens  has  been 
shown  to  be  the  same  thing  in  some  respects  as  removing  it  to  a 
greater  distance.  We  have  to  see  the  magnified  picture  by  the 
light  emanating  from  the  small  object,  and  it  becomes  a  matter 
of  difficulty  to  obtain  from  each  point  a  sufficient  quantity  of 
light  to  bear  the  diffusion  of  a  great  magnifying  power.  We 
want  to  perform  an  operation  just  the  reverse  of  applying  the 
card  with  the  needle-hole  to  the  eye — we  want  in  some  cases  to 
bring  into  the  eye  the  largest  possible  pencil  of  light  from  each 
point  of  the  object. 

Eeferring  to  Fig.  3,  it  will  be  observed  that  if  the  eye  could 
see  the  small  arrow  at  the  distance  there  shown  without  the 
intervention  of  the  lens,  only  a  very  small  portion  of  the  cones 
of  light  drawn  from  its  extremities  would  enter  the  pupil; 
whereas  we  have  supposed  that  after  being  bent  by  the  lens  the 
whole  of  this  light  enters  the  eye  as  part  of  the  cones  of  smaller 
angle  whose  summits  are  at  C  and  D.  These  cones  will  further 


10  THE    MICROSCOPE. 

explain  the  difference  between  large  and  small  pencils  of  light; 
those  from  the  small  arrow  are  large  pencils;  the  dotted  cones 
from  the  large  arrow  are  small  pencils. 

In  assuming  that  the  whole  of  this  light  could  have  been  suf- 
fered to  enter  the  eye  through  the  lens  A  B,  we  did  so  for  the 
sake  of  not  perplexing  the  reader  with  too  many  considerations 
at  once.  He  must  now  learn  that  so  large  a  pencil  of  light 
passing  through  a  single  lens  would  be  so  distorted  by  the 
spherical  figure  of  the  lens,  and  by  the  chromatic  dispersion  of 
the  glass,  as  to  produce  a  very  confused  and  imperfect  image. 
This  confusion  may  be  greatly  diminished  by  reducing  the 
pencil;  for  instance,  by  applying  a  stop,  as  it  is  called,  to  the 
lens,  which  is  neither  more  nor  less  than  the  needle-hole  ap- 
plied to  the  eye.  A  small  pencil  of  light  may  be  thus  trans- 
mitted through  a  single  lens  without  suffering  from  spherical 
aberration  or  chromatic  dispersion  any  amount  of  distortion 
which  will  materially  affect  the  figure  of  the  object;  but  this 
quantity  of  light  is  insufficient  to  bear  diffusion  over  the  mag- 
nified picture,  which  is  therefore  too  obscure  to  exhibit  what 
we  most  desire  to  see — those  beautiful  and  delicate  markings 
by  which  one  kind  of  organic  matter  is  distinguished  from 
another.  With  a  small  aperture  these  markings  are  not  seen  at 
all:  with  a  large  aperture  and  a  single  lens  they  exhibit  a  faint 
nebulous  appearance  enveloped  in  a  chromatic  mist,  a  state 
which  is  of  course  utterly  valueless  to  the  naturalist,  and  not 
even  amusing  to  the  amateur. 

It  becomes  therefore  a  most  important  problem  to  reconcile  a 
large  aperture  with  distinctness,  or,  as  it  is  called,  denfinition; 
and  this  has  been  done  in  a  considerable  degree  by  effecting 
the  required  amount  of  refraction  through  two  or  more  lenses 
instead  of  one,  thus  reducing  the  angles  of  incidence  and  re- 
fraction, and  producing  other  effects  which  will  be  shortly 
noticed.  This  was  first  accomplished  in  a  satisfactory  manner 
by- 

DR.  WOLLASTON'S  DOUBLET, 

invented  by  the  celebrated  philosopher  whose  name  it  bears; 
it  consists  of  two  plano-convex  lenses  (Fig.  4)  having  their 
focal  lengths  in  the  proportion  of  1  to  3,  or  nearly  so,  and 


THE    MICKOSCOPE.  11 

placed  at  a  distance  which  can  be  ascertained  best  by  actual 
experiment.  Their  plane  sides  are  placed  towards  the  object, 
and  the  lens  of  shortest  focal  length  next  the  object. 

It  appears  that  Dr.  Wollaston  was  led  to  this  invention  by 

considering  that  the  Achromatic  Huyghenean  Eye-piece,  which 

will  be  hereafter  described,  would,  if  reversed,  possess  similar 

good  properties  as  a  simple  microscope.     But  it  will  be  evident 

when  the    eye-piece  is  understood,   that    the    circumstances 

which  render  it  achromatic  are  very  imperfectly 

^  applicable  to  the  simple  microscope,  and  that  the 

doublet,  without  a  nice  adjustment  of  the  stop, 

^-«r        would    be    valueless.     Dr.    Wollaston    makes    no 

Fig.  4.  allusion  to  a  stop,  nor  is  it  certain  that  he  contem- 
plated its  introduction,  although  his  illness,  which 
terminated  fatally  soon  after  the  presentation  of  his  paper, 
may  account  for  the  omission. 

The  nature  of  the  corrections  which  take  place  in  the 
doublet  is  explained  in  the  annexed  diagram  (Fig.  5),  where 
L  O  L'  is  the  object,  P  a  portion  of  the  pupil,  and  D  D  the 
stop,  or  limiting  aperture. 

Now,  it  will  be  observed  that  each  of  the  pencils  of  light 
from  the  extremities  L  L'  of  the  object  is  rendered  eccentrical 
by  the  stop,  and  of  consequence  each  passes  through  the  two 
lenses  on  opposite  sides  of  their  common  axis  O  P;  thus  each 
becomes  affected  by  opposite  errors,  which  to  some  extent 
balance  and  correct  each  other.  To  take  the  pencil  L,  for 
instance,  which  enters  the  eye  at  R  B,  B,  B ;  it  is  bent  to  the 
right  at  the  first  lens,  and  to  the  left  at  the  second;  and  as  each 
bending  alters  the  direction  of  the  blue  rays  more  than  the 
red,  and,  moreover,  as  the  blue  rays  fall  nearer  the  margin  of 
the  second  lens,  where  the  refraction,  being  more  powerful  than 
near  the  centre,  compensates  in  some  degree  for  the  greater 
focal  length  of  the  second  lens,  the  blue  and  red  rays  will 
emerge  very  nearly  parallel,  and  of  consequence  colorless  to 
the  eye.  At  the  same  time  the  spherical  aberration  has  been 
diminished  by  the  circumstance  that  the  side  of  the  pencil 
which  passes  one  lens  nearest  the  axis  passes  the  other  nearest 
the  margin. 

This  explanation  applies  only  to  the  pencils  near  the  extrem- 


12 


THE    MICKOSCOPE. 


ities  of  the  object.  The  central  pencil,  it  is  obvious,  would 
pass  both  .lenses  symmetrically;  the  same  portions  of  light 
occupying  nearly  the  same  relative  places  on  both  lenses.  The 
blue  light  would  enter  the  second  lens  nearer  to  its  axis  than 
the  red,  and  being  thus  less  refracted  than  the  red  by  the 


second  lens,  a  small  amount  of  compensation  would  take  place, 
quite  different  in  principle  and  inferior  in  degree  to  that  which 
is  produced  in  the  eccentrical  pencils.  In  the  intermediate 
spaces  the  corrections  are  still  more  imperfect  and  uncertain; 
and  this  explains  the  cause  of  the  aberrations  which  must  of 
necessity  exist  even  in  the  best-made  doublet.  It  is,  however, 
infinitely  superior  to  a  single  lens,  and  will  transmit  a  pencil  of 
an  angle  of  from  35°  to  50°  without  any  very  sensible  errors. 


THE    MICROSCOPE.  13 

It  exhibits,  therefore,  many  of  the  usual  test-objects  in  a  very 
beautiful  manner. 

The  next  step  in  the  improvement  of  the  simple  microscope 
bears  more  analogy  to  the  eye-piece.  This  improvement  was 
made  by  Mr.  Holland,  and  it  consists  (as  shown  in  Fig.  6)  in 
substituting  two  lenses  for  the  first  in  the  doublet,  and  retain- 
ing the  stop  between  them  and  the  third.  The  first  bending, 
being  thus  effected  by  two  lenses  instead  of 
one,  is  accompanied  by  smaller  aberrations, 
which  are  therefore  more  completely  balanced 
or  corrected  at  the  second  bending,  in  the 
Fig.  6.  opposite  direction,  by  the  third  lens.  This 

combination,  though  called  a  triplet  is  essen- 
tially a  doublet,  in  which  the  anterior  lens  is  divided  into  two. 
For  it  must  be  recollected  that  the  first  pair  of  lenses  merely 
accomplishes  what  might  have  been  done,  though  with  less 
precision,  by  one;  but  the  two  lenses  of  the  doublet  are  opposed 
to  each  other;  the  second  diminishing  the  magnifying  power 
of  the  first.  The  first  pair  of  lenses  in  the  triplet  concur  in 
producing  a  certain  amount  of  magnifying  power,  which  is 
diminished  in  quantity  and  corrected  as  to  aberration  at  the 
third  lens  by  the  change  in  relation  to  the  position  of  the  axis 
which  takes  place  in  the  pencil  between  what  is  virtually  the 
first  and  second  lens.  In  this  combination  the  errors  are  still 
further  reduced  by  the  close  approximation  to  the  object  which 
causes  the  refractions  to  take  place  near  the  axis.  Thus  the 
transmission  of  a  still  larger  angular  pencil,  namely  65°,  is  ren- 
dered compatible  with  distinctness,  and  a  more  intense  image 
is  presented  to  the  eye. 

Every  increase  in  the  number  of  lenses  is  attended  with  one 
drawback,  from  the  circumstance  that  &  certain  portion  of 
light  is  lost  by  reflection  and  absorption  each  time  that  the  ray 
enters  a  new  medium.  This  loss  bears  no  sensible  proportion 
to  the  gain  arising  from  the  increased  aperture,  which,  being 
as  the  square  of  the  diameter,  multiplies  rapidly;  or,  if  we  esti- 
mate by  the  angle  of  the  admitted  pencil,  which  is  more  easily 
ascertained,  the  intensity  will  be  as  the  square  of  twice  the  tan- 
gent of  half  the  angle.  To  explain  this,  let  D  B  (Fig.  7)  repre- 
sent the  diameter  of  the  lens,  or  of  that  part  of  it  which  is 


MIOKOSOOPE. 


really  employed;  0  A  the  perpendicular  drawn  from  its  centre, 
and  A  JB,  A  D,  the  extreme  rays  of  the  incident  pencil  of 
light  DAB.  Then  the  diameter  being  2  C  B,  the  area  to 
which  the  intensity  of  vision  is  proportional  will  be  (2  C  B)2, 
and  0  B  is  evidently  the  tangent  of  the  angle  CAB,  which  is 
half  the  angle  of  the  admitted  pencil  DAB. 
Or,  if  a  be  used  to  denote  the  angular  aper- 
ture, the  expression  for  the  intensity  is  (2 
tan.  i«)2,  which  increases  so  rapidly  with  the 
increase  of  a  as  to  make  the  loss  of  light  by 
reflection  and  absorption  of  little  conse- 
quence. 

The  combination  of  three  lenses  ap- 
proaches, as  has  been  stated,  very  close  to 
the  object;  so  close,  indeed,  as  to  prevent 
the  use  of  more  than  three;  and  this  consti- 
tutes a  limit  to  the  improvement  of  the  sim- 
ple microscope,  for  it  is  called  a  simple  microscope,  although 
consisting  of  three  lenses,  and  although  a  compound  micro- 
scope may  be  made  of  only  three  or  even  two  lenses;  but  the 
different  arrangement  which  gives  rise  to  the  term  compound 
will  be  better  understood  when  that  instrument  is  explained. 

Before  we  proceed  to  describe  the  simple  microscope  and  its 
appendages,  it  will  be  well  to  explain  such  other  points  in  refer- 
ence to  the  form  and  materials  of  lenses  as  are  most  likely  to 
be  interesting. 

A  very  useful  form  of  lens  was  proposed  by  Dr.  Wollaston, 
and  called  by  him  the  Periscopic  lens.  It  consisted  of  two 
hemispherical  lenses,  cemented  together  by  their  plane  faces, 
having  a  stop  between  them  to  limit  the  aperture.  A  similar 
proposal  was  made  by  Mr.  Coddington,  who,  however,  executed 
the  project  in  a  better  manner,  by  cutting  a  groove  in  a  whole 
sphere,  and  filling  the  groove  with  opaque  matter.  His  lens, 
which  is  the  well-known  Coddington  lens,  is  shown  in  Fig.  8. 
It  gives  a  large  field  of  view,  which  is  equally  good  in  all  direc- 
tions, as  it  is  evident  that  the  pencils  A  A  and  B  B  pass  through 
under  precisely  the  same  circumstances.  Its  spherical  form  has 
the  further  advantage  of  rendering  the  position  in  which  it  is 
held  of  comparatively  little  consequence.  It  is  therefore  very 


THE    MICROSCOPE. 


15 


convenient  as  a  hand-lens,  but  its  definition  is  of 
course  not  so  good  as  that  of  a  well-made 
doublet  or  achromatic  lens. 

Another  very  useful  form  of  doublet  was  pro- 
posed by  Sir  John  Herschel,  chiefly  like  the 
Coddington  lens,  for  the  sake  of  a  wide  field,  and 
chiefly  to  be  used  in  the  hand.  It  is  shown  in 
Fig.  9;  it  consists  of  a  double  convex  or  crossed 
lens,  having  the  radii  of  curvature  as  1  to  6,  and 
of  a  plane  concave  lens  whose  focal  length  is  to 
that  of  the  convex  lens  as  13  to  5. 

Various,  indeed  innumerable,  other  forms  and 
combinations  of  lenses  have  been  projected, 
some  displaying  much  ingenuity,  but  few  of  any 
practical  use.  In  the  Catadioptric  lenses  the 
light  emerges  at  right  angles  from  its  entering 
direction,  being  reflected  from  a  surface  cut  at  an 
angle  of  45  degrees  to  the  axes  of  the  curved 
surfaces.  A  ** 

It  was  at  one  time  hoped,  as  the  precious          F*g-  8> 
stones  are  more  refractive  than  glass,  and  as  the 
increased  refractive  power  is  unaccompanied  by  a  correspondent 
increase  in  chromatic    dispersion,   that    they  would  furnish 
valuable  materials  for  lenses,  inasmuch  as  the  refractions  would 
be  accomplished  by  shallower  curves,  and  consequently  with 
diminished  spherical  aberration.     But  these  hopes  were  disap- 
pointed; everything  that  ingenuity  and   perseverance  could 

accomplish  was  tried  by  Mr. 
Varley  and  Mr.  Pritchard,  under 
the  patronage  of  Dr.  Goring. 

~   It  appeared,  however,  that  the 

great  reflective  power,  the 
doubly-refracting  property,  the 
color,  and  the  heterogeneous 
structure  of  the  jewels  which 
were  tried,  much  more  than 

counterbalanced  the  benefits  arising  from  their  greater  refrac- 
tive power,  and  left  no  doubt  of  the  superiority  of  skillfully 
made  glass  doublets  and  triplets.  The  idea  is  now,  in  fact, 


Fig.  9. 


THE    MICROSCOPE. 

abandoned;  and  the  same  remark  is  applicable  to  the  attempts 
at  constructing  fluid  lenses,  and  to  the  projects  for  giving  to 
glass  other  than  spherical  surfaces — none  of  which  have  come 
into  extensive  use. 

By  the  term  simple  microscope  is  meant  one  in  which  the 
object  is  viewed  directly  through  a  lens  or  combination  of 
lenses,  just  as  we  have  supposed  an  arrow  or  an  insect  to  be 
viewed  through  a  glass  held  in  the  hand.  When,  however,  the 
magnifying  power  of  the  glass  is  considerable,  in  other  words, 
when  its  focal  length  is  very  short,  and  its  proper  distance  from 
its  object  of  consequence  equally  short,  it  requires  to  be  placed 
at  that  proper  distance  with  great  precision:  it  cannot,  there- 
fore, be  held  with  sufficient  accuracy  and  steadiness  by  the  un- 
assisted hand,  but  must  be  mounted  in  a  frame  having  a  rack 
or  screw  to  move  it  towards  or  from  another  frame  or  stage 
which  holds  the  object.  It  is  then  called  a  microscope,  and  it 
is  furnished,  according  to  circumstances,  with  lenses  and  mir- 
rors to  collect  and  reflect  the  light  upon  the  object,  and  with 
other  conveniences  which  will  now  be  described. 

One  of  the  best  forms  of  a  stand  for  a  simple  microscope  is 
shown  in  Fig.  10,  where  A  is  a  brass  pillar  screwed  to  a  tripod 
base;  B  is  a  broad  stage  for  the  objects,  secured  to  the  stem  by 
screws,  whose  milled  heads  are  at  0.  By  means  of  the  large 
milled  head  D,  a  triangular  bar,  having  a  rack,  is  elevated  out 
of  the  stem  A,  carrying  the  lens-holder  E,  which  has  a  hori- 
zontal movement  in  one  direction,  by  means  of  .a  rack  worked 
by  the  milled  head  F,  and  in  the  other  direction  by  turning  on 
a  circular  pin.  A  concave  mirror  G  reflects  the  light  upwards 
through  the  hole  in  the  stage,  and  a  lens  may  be  attached  to 
the  stage  for  the  purpose  of  throwing  light  on  an  opaque  ob- 
ject, in  the  same  way  that  the  forceps  H  for  holding  such  ob- 
jects is  attached.  This  microscope  is  peculiarly  adapted,  by  its 
broad  stage  and  its  general  steadiness,  for  dissecting;  and  it  is 
rendered  more  convenient  for  this  purpose  by  placing  it  be- 
tween two  inclined  planes  of  mahogany,  which  support  the 
arms  and  elevate  the  wrists  to  the  level  of  the  stage.  This  ap- 
paratus is  called  the  dissecting  rest.  When  dissecting  is  not  a 
primary  object,  a  joint  may  be  made  at  the  lower  end  of  the 
stem  A,  to  allow  the  whole  to  take  an  inclined  position;  and 


THE    MICROSCOPE. 


17 


then  the  spring  clips  shown  upon  the  stage  are  useful  to  retain 
the  object  in  its  place.  Numerous  convenient  appendages  may 
be  made  to  accompany  such  microscopes,  which  it  will  be  im- 
possible to  mention  in  detail;  the  most  useful  are  Mr.  Varley's 
capillary  cages  for  containing  animalculae  in  water,  and  parts  of 
aquatic  plants;  also  his  tubes  for  obtaining  and  separating  such 


Jb';g.  10. 

objects,  and  his  phial  and  phial-holder  for  preserving  and  ex- 
hibiting small  living  specimens  of  the  Chara,  Nitella,  and  other 
similar  plants,  and  observing  their  circulation.  The  phial- 
microscope  affords  facilities  for  observing  the  operations  of 
minute  vegetable  and  animal  life,  which  will  probably  lead 
to  the  most  interesting  discoveries.  The  recent  volumes  of 
the  Transactions  of  the  Society  of  Arts  contain  an  immense 


18  THE    MIOKOSCOPE. 

mass  of  information  of  this  sort,  and  to  these  we  refer  the 
reader. 

The  mode  of  illuminating  objects  is  one  on  which  we  must 
give  some  further  information,  for  the  manner  in  which  an  ob- 
ject is  lighted  is  second  in  importance  only  to  the  excellence 
of  the  glass  through  which  it  is  seen.  In  investigating  any 
new  or  unknown  specimen,  it  should  be  viewed  in  turns  by 
every  description  of  light,  direct  and  oblique,  as  a  transparent 
object  and  as  an  opaque  object,  with  strong  and  with  faint 
light,  with  large  angular  pencils  and  with  small  angular  pencils 
thrown  in  all  possible  directions.  Every  change  will  probably 
develop  some  new  fact  in  reference  to  the  structure  of  the  ob- 
ject, which  should  itself  be  varied  in  the  mode  of  mounting  in 
every  possible  way.  It  should  be  seen  both  wet  and  dry,  and 
immersed  in  fluids  of  various  qualities  and  densities,  such  as 
water,  alcohol,  oil,  and  Canada  balsam,  for  instance,  which  last 
has  a  refractive  power  nearly  equal  to  that  of  glass.  If  the 
object  be  delicate  vegetable  tissue,  it  will  be  in  some  respects 
rendered  more  visible  by  gentle  heating  or  scorching  by  a  clear 
fire  placed  between  two  plates  of  glass.  In  this  way  the  spiral 
vessels  of  asparagus  and  other  similar  vegetables  may  be  beau- 
tifully displayed.  Dyeing  the  objects  in  tincture  of  iodine  will 
in  some  cases  answer  this  purpose  better. 

Bat  the  principal  question  in  regard  to  illumination  is  the 
magnitude  of  the  illuminating  pencil,  particularly  in  reference 
to  transparent  objects.  Generally  speaking  the  illuminating 
pencil  should  be  as  large  as  can  be  received  by  the  lens,  and  no 
larger.  Any  light  beyond  this  produces  indistinctness  and 
glare.  The  superfluous  light  from  the  mirror  may  be  cut  off 
by  a  screen  having  various-sized  apertures  placed  below  the 
stage;  but  the  best  mode  of  illumination  is  that  proposed  by 
Dr.  Wollaston,  and  called  the  Wollaston  condenser.  A  tube  is 
placed  below  the  stage  of  the  instrument  containing  a  lens  A  B 
(Fig.  11),  which  can  be  elevated  or  depressed  within  certain 
limits  at  pleasure;  and  at  the  lower  end  is  a  stop  with  a  limited 
aperture  CD.  A  plane  mirror  E  F  receives  the  rays  of  light 
L  L  from  the  sky  or  a  white  cloud,  which  last  is  the  best  source 
of  light,  and  reflects  them  upwards  through  the  aperture  in 
C  D,  so  that  they  are  refracted,  and  form  an  image  of  the 


THE    MICROSCOPE. 


19 


aperture  at  G,  which  is  supposed  to  be  nearly  the  place  of  the 
object.  The  object  is  sometimes  best  seen  when  the  image  of 
the  aperture  is  also  best  seen;  and  sometimes  it  is  best  to  ele- 
vate the  summit  G  of  the  cone  A  B  G  above  the  object,  and  at 
others  to  depress  it  below :  all  which  is  done  at  pleasure  by  the 
power  of  moving  the  lens  A  B.  If  artifical  light  (as  a  lamp  or 
candle)  be  employed,  the  flame  must  be  placed  in  the  principal 
focus  of  a  large  detached  lens  on  a 
stand,  so  that  the  rays  L  L  may  fall  in 
parallel  lines  on  the  mirror,  or  as  they 
would  fall  from  the  cloud.  This  will  be 
found  an  advantage,  not  only  when  the 
Wollaston  condenser  is  employed,  but 
also  when  the  mirror  and  diaphragm  are 
used.  A  good  mode  of  imitating  arti- 
ficially the  light  of  a  white  cloud  op- 
posite the  sun  has  been  proposed  by 
Mr.  Yarley;  he  covers  the  surface  of  the 
mirror  under  the  stage  with  carbon- 
ate of  soda  or  any  similar  material, 
and  then  concentrates  the  sun's  light 
upon  its  surface  by  a  large  condensing 
lens.  The  intense  whi£e  light  diffused 
from  the  surface  of  the  soda  forms  an 
excellent  substitute  for  the  white  cloud, 
which,  when  opposite  the  sun,  and  of 
considerable  size,  is  the  best  daylight,  as 
the  pure  sky  opposite  to  the  sun  is  the 
worst. 

The  Compound  Microscope  may,  as  be- 
fore stated,  consist  of  only  two  lenses, 

while  a  simple  microscope  has  been  shown  to  contain  sometimes 
three.  In  the  triplet  for  the  simple  microscope,  however,  it 
was  explained  that  the  effect  of  the  two  first  lenses  was  to  do 
what  might  have  been  accomplished,  though  not  so  well,  by 
one;  and  the  third  merely  effected  certain  modifications  in  the 
light  before  it  entered  the  eye.  But  in  the  compound  micro- 
scope the  two  lenses  have  totally  different  functions;  the  first 
receives  the  rays  from  the  object,  and,  bringing  them  to  new 


Fig.  11. 


20 


THE    MICROSCOPE. 

foci,  forms  an  image,  which  the  second 
lens  treats  as  an  original  object,  and  mag- 
nifies it  just  as  the  single  microscope  mag- 
nified the  object  itself. 

The  annexed  figure  (12)  shows  the 
course  of  the  rays  through  a  compound 
microscope  of  two  lenses.  The  rays  pro- 
ceeding from  the  object  A  B  are  so  acted 
upon  by  the  lens  C  D,  near  it,  and  thence 
called  the  object  glass,  that  they  are  con- 
verged to  foci  in  A'  B',  where  they  form 
an  enlarged  image  of  the  object,  as  would 
be  evident  if  a  piece  of  oiled  paper  or 
ground  glass  were  placed  there  to  receive 
them.  They  are  not  so  intercepted,  and 
therefore  the  image  is  not  rendered  visi- 
ble at  that  place;  but  their  further  pro- 
gress is  similar  to  what  it  would  have  been 
had  they  really  proceeded  from  an  object 
at  A'  B'.  They  are  at  length  received  by 
the  eye-lens  L  M,  which  acts  upon  them 
as  the  simple  microscope  has  been  de- 
scribed to  act  on  the  light  proceeding 
from  its  objects.  They  are  bent  so  that 
they  may  enter  the  eye  at  E  in  parallel 
lines,  or  as  nearly  so  as  is  requisite  for 
distinct  vision.  When  we  say  that  the 
rays  enter  the  eye  in  nearly  parallel  lines, 
we  mean  only  those  which  proceed  from 
one  point  of  the  original  object.  Thus 
the  two  parallel  rays  M  E  have  proceeded 
from  and  are  part  of  the  cone  of  rays 
CAD,  emanating  from  the  point  A  of 
the  arrow;  but  they  do  not  form  two  pic- 
•  tures  in  the  eye,  because  any  number  of 
parallel  rays  which  the  pupil  can  receive 
will  be  converged  to  a  point  by  the  eye, 
and  will  convey  the  impression-  of  one 
point  to  the  mind.  In  like  manner  the 


THE    MICBOSCOPE.  21 

rays  L  E  are  part  of  the  cone  of  rays  emanating  from  B,  and 
the  angle  L  E  M  is  that  under  which  the  eye  will  see  the  mag- 
nified image  of  the  arrow,  which  is  evidently  many  times 
greater  than  the  arrow  could  be  made  to  occupy  in  the  naked 
eye  at  any  distance  within  the  limits  of  distinct  vision.  The 
magnifying  power  depends  on  two  circumstances:  first,  on  the 
ratio  between  the  anterior  distance  A  0  or  B  D  and  the  poste- 
rior focal  length  C  B'  or  D  A';  and  secondly,  on  the  power  of 
the  eye-lens  L  M.  The  first  ratio  is  the  same  as  that  between 
the  object  A  B  and  the  image  A'  B';  this  and  the  focal  length 
or  power  of  the  eye  lens  are  both  easily  obtained,  and  their 
product  is  the  power  of  the  compound  instrument. 

Since  the  power  depends  on  the  ratio  between  the  anterior 
and  posterior  foci  of  the  object-glass,  it  is  evident  that  by  in- 
creasing that  ratio  any  power  may  be  obtained,  the  same  eye- 
glass being  used ;  or  having  determined  the  first,  any  further 
power  may  be  obtained  by  increasing  that  of  the  eye-glass;  and 
thus,  by  a  pre-arrangement  of  the  relative  proportions  in  which 
the  magnifying  power  shall  be  divided  between  the  object-glass 
and  the  eye-glass,  almost  any  given  distance  (within  certain 
limits)  between  the  first  and  its  object  may  be  secured.  This 
is  one  valuable  peculiarity  of  the  compound  instrument;  and 
another  is  the  large  field,  or  large  angle  of  view,  which  may  be 
obtained,  every  part  of  which  will  be  nearly  equally  good; 
whereas  with  the  best  simple  microscopes  the  field  is  small,  and 
is  good  only  in  the  centre.  The  field  of  the  compound  instru- 
ment is  further  increased  by  using  two  glasses  at  the  eye-end; 
the  first  being  called,  from  its  purpose,  the  field-glass,  and  the 
two  constituting  what  is  called  the  eye-piece.  This  will  be 
more  particularly  explained  in  the  figure  of  the  achromatic 
compound  microscope  presently  given. 

For  upwards  of  a  century  the  compound  microscope,  not- 
withstanding the  advantages  above  mentioned,  was  a  compara- 
tively feeble  and  inefficient  instrument,  owing  to  the  distance 
which  the  light  had  to  traverse,  and  the  consequent  increase  of 
the  chromatic  and  spherical  aberrations.  To  explain  this  we 
have  drawn  in  Fig.  12  a  second  image  near  A'  B',  the  fact  being 
that  the  object-glass  would  not  form  one  image,  as  has  been 
supposed,  but  an  infinite  number  of  vaiiously-colored  and  vari- 


22  THE    MIOBOSCOPE. 

ous-sized  images,  occupying  the  space  between  the  two  dotted 
arrows.  Those  nearest  the  object-glass  would  be  red,  and  those 
nearest  the  eye-glass  would  be  blue.  The  effect  of  this  is  to 
produce  so  much  confusion,  that  the  instrument  was  reduced 
to  a  mere  toy,  although  these  errors  were  diminished  to  the 
utmost  possible  extent  by  limiting  the  aperture  of  the  object- 
glass,  and  thus  restricting  the  angle  of  the  pencil  of  light  from 
each  point  of  the  object.  But  this  involved  the  defects,  already 
explained,  of  making  the  picture  obscure,  so  that  on  the  whole 
the  best  compound  instruments  were  inferior  to  the  simple 
microscopes  of  a  single  lens,  with  which,  indeed,  all  the  impor- 
tant observations  of  the  last  century  were  made. 

Even  after  the  improvement  of  the  simple  microscope  by  the 
use  of  doublets  and  triplets,  the  long  course  of  the  rays,  and 
the  large  angular  pencil  required  in  the  compound  instrument, 
deterred  the  most  sanguine  from  anticipating  the  period  when 
they  should  be  conducted  through  such  a  path  free  both  from 
spherical  and  chromatic  errors.  Within  twenty  years  of  the 
present  period,  philosophers  of  no  less  eminence  than  M.  Blot 
and  Dr.  Wollaston  predicted  that  the  compound  would  never 
rival  the  simple  microscope,  and  that  the  idea  of  achromatizing 
its  object-glass  was  hopeless.  Nor  can  these  opinions  be  won- 
dered at  when  we  consider  how  many  years  the  achromatic 
telescope  had  existed  without  an  attempt  to  apply  its  principles 
to  the  compound  microscope.  When  we  consider  the  smallness 
of  the  pencil  required  by  the  telescope,  and  the  enormous  in- 
crease of  difficulty  attending  every  enlargement  of  the  pencil — 
when  we  consider  further  that  these  difficulties  had  to  be  con- 
tended with  and  removed  by  operations  on  portions  of  glass  so 
small  that  they  are  themselves  almost  microscopic  objects,  we 
shall  not  be  surprised  that  even  a  cautious  philosopher  and  most 
able  manipulator  like  Dr.  Wollaston  should  prescribe  limits  to 
improvement. 

Fortunately  for  science,  and  especially  for  the  departments 
of  animal  and  vegetable  physiology,  these  predictions  have 
been  shown  to  be  unfounded.  The  last  fifteen  years  have  suf- 
ficed to  elevate  the  compound  microscope  from  the  condition 
we  have  described  to  that  of  being  the  most  important  instru- 
ment ever  bestowed  by  art  upon  the  investigator  of  nature.  It 


THE    MJCKOSCOPE.  23 

now  holds  a  very  high  rank  among  philosophical  implements, 
while  the  transcendant  beauties  of  form,  color  and  organization, 
which  it  reveals  to  us  in  the  minute  works  of  nature,  render  it 
subservient  to  the  most  delightful  and  instructive  pursuits. 
To  these  claims  on  our  attention,  it  appears  likely  to  add  a 
third  of  still  higher  importance.  The  microscopic  examination 
of  the  blood  and  other  human  organic  matter  will  in  all  proba- 
bility afford  more  satisfactory  and  conclusive  evidence  regard- 
ing the  nature  and  seat  of  disease  than  any  hitherto  appealed 
to,  and  will  of  consequence  lead  to  similar  certainty  in  the 
choice  and  application  of  remedies. 

We  have  thought  it  necessary  to  state  thus  at  large  the  claims 
of  the  modern  achromatic  microscope  upon  the  attention  of  the 
reader,  as  a  justification  of  the  length  at  which  we  shall  give 
its  recent  history  and  explain  its  construction;  and  we  are  fur- 
ther induced  to  this  course  by  the  consideration  that  the  sub- 
ject is  entirely  new  ground,  and  that  there  are  at  this  time  not 
more  than  two  or  three  makers  of  achromatic  microscopes  in 
England. 

Soon  after  the  year  1820  a  series  of  experiments  was  begun 
in  France  by  M.  Selligues,  which  were  followed  up  by  Frauen- 
hofer  at  Munich,  by  Amici  at  Modena,  by  M.  Chevalier  at 
Paris,  and  by  the  late  Mr.  Tulley  in  London.  In  1824  the  last- 
named  excellent  artist,  without  knowing  what  had  been  done 
on  the  Continent,  made  the  attempt  to  construct  an  achromatic 
object-glass  for  a  compound  microscope,  and  produced  one  of 
nine-tenths  of  an  inch  focal  length,  composed  of  three  lenses, 
and  transmitting  a  pencil  of  eighteen  degrees.  This  was  the 
first  that  had  been  made  in  England;  and  it  is  due  to  Mr.  Tul- 
ley to  say,  that  as  regards  accurate  correction  throughout  the 
field,  that  glass  has  not  been  excelled  by  any  subsequent  com- 
bination of  three  lenses.  Such  an  angular  pencil,  and  such  a 
focal  length,  would  bear  an  eye-piece  adapted  to  produce  a 
gross  magnifying  power  of  one  hundred  and  twenty.  Mr.  Tul- 
ley afterwards  made  a  combination  to  be  placed  in  front  of  the 
first  mentioned,  which  increased  the  angle  of  the  transmitted 
pencil  to  thirty-eight  degrees,  and  bore  a  power  of  three  hun- 
dred. 

While  these  practical  investigations  were  in  progress,  the 


24:  THE    MIOBOSCOPE. 

subject  of  achromatism  engaged  the  attention  of  some  of  the 
most  profound  mathematicians  in  England.  Sir  John  Herschel, 
Professor  Airy,  Professor  Barlow,  Mr.  Coddington,  and  others, 
contributed  largely  to  the  theoretical  examination  of  the  sub- 
ject; and  though  the  results  of  their  labors  were  not  immedi- 
ately applicable  to  the  microscope,  they  essentially  promoted 
its  improvement. 

For  some  time  prior  to  1829  the  subject  had  occupied  the 
mind  of  a  gentleman,  who,  not  entirely  practical,  like  the  first, 
nor  purely  mathematical,  like  the  last-mentioned  class  of  in- 
quirers, was  led  to  the  discovery  of  certain  properties  in 
achromatic  combinations  which  had  been  before  unobserved. 
These  were  afterwards  experimentally  verified;  and  in  the  year 
1829  a  paper  on  the  subject,  by  the  discoverer,  Mr.  Joseph 
Jackson  Lister,  was  read  and  published  by  the  Royal  Society. 
The  principles  and  results  thus  obtained  enabled  Mr.  Lister  to 
form  a  combination  of  lenses  which  transmitted  a  pencil  of 
fifty  degrees,  with  a  large  field  correct  in  every  part;  as  this 
paper  was  the  foundation  of  the  recent  improvements  in 
acromatic  microscopes,  and  as  its  results  are  indispensable  to 
all  who  would  make  or  understand  the  instrument,  we  shall  give 
the  more  important  parts  of  it  in  detail,  and  in  Mr.  Lister's 
own  words. 

"  I  would  premise  that  the  plano-concave  form  for  the  cor- 
recting flint  lens  has  in  that  quality  a  strong  recommendation, 
particularly  as  it  obviates  the  danger  of  error  which  otherwise 
exists  in  centering  the  two  curves,  and  thereby  admits  of  cor- 
rect workmanship  for  a  shorter  focus.  To  cement  together 
also  the  two  surfaces  of  the  glass  diminishes  by  very  nearly 
half  the  loss  of  light  from  reflection,  which  is  considerable  at 
the  numerous  surfaces  of  a  combination.  I  have  thought  the 
clearness  of  the  field  and  brightness  of  the  picture  evidently 
increased  by  doing  this;  it  prevents  any  dewiness  or  vegetation 
from  forming  on  the  inner  surfaces;  and  I  see  no  disadvantage 
to  be  anticipated  from  it  if  they  are  of  identical  curves,  and 
pressed  closely  together,  and  the  cementing  medium  per- 
manently homogeneous. 

"  These  two  conditions  then,  that  the  flint  lens  shall  be  plano- 
concave, and  that  it  shall  be  joined  by  some  cement  to  the  con- 


THE    MICROSCOPE. 


26 


vex,  seem  desirable  to  be  taken  as  a  basis  for  the  microscopic 
object-glass,  provided  they  can  be  reconciled  with  the  destruc- 
tion of  the  spherical  and  chromatic  aberrations  of  a  large 
pencil. 

* '  Now  in  every  such  glass  that  has  been  tried  by  me  which 
has  had  its  correcting  lens  of  either  Swiss  or  English  glass, 
with  a  double  convex  of  plate,  and  has  been  made  achromatic 
by  the  form  given  to  the  outer  curve  of  the  convex,  the  pro- 
portion has  been  such  between  the  refrac- 
CJ       H  tive  and  dispersive  powers  of  its  lenses,  that 
its  figure  has  been  correct  for  rays  issuing 
from  some  point  in  its  axis  not  far  from  its 
principal  focus  on  its  plane  side,  and  either 
tending  to  a  conjugate  focus  within  the  tube 
of  a  microscope,  or  emerging  nearly  par- 
allel. 

"  Let  A  B  (Fig.  13)  be  supposed  such  an 
object-glass,  and  let  it  be  roughly  considered 
as  a  plano-convex  lens,  with  a  curve  A  G  B 
running  through  it,  at  which  the  spherical 
and  chromatic  errors  are  corrected  which  are 
generated  at  the  two  outer  surfaces;  and  let 
the  glass  be  thus  free  from  aberration  for 
rays  F  D  E  G  issuing  from  the  radiant 
point  F,  H  E  being  a  perpendicular  to  the 
convex  surface,  and  I  D  to  the  plane  one. 
Under  these  circumstances,  the  angle  of 
emergence  G  E  H  much  exceeds  that  of  in- 
cidence F  D  I,  being  probably  nearly  three 
times  as  great. 

"If  the  radiant  is  now  made  to  approach 
the  glass,  so  that  the  course  of  the  ray  F  D 
E  G  shall  be  more  divergent  from  the  axis,  as  the  angles  of  in- 
cidence and  emergence  become  more  nearly  equal  to  each 
other,  the  spherical  aberration  produced  by  the  two  will  be 
found  to  bear  a  less  proportion  to  the  opposing  error  of  the 
single  correcting  curve  A  C  B;  for  such  a  focus  therefore  the 
rays  will  be  over-corrected. 

"But  if  F  still  approaches  the  glass,  the  angle  of  incidence 


Fig.  13. 


26  THE    MICEOSCOPE. 

continues  to  increase  with  the  increasing  divergence  of  the  ray, 
till  it  will  exceed  that  of  emergence,  which  has  in  the  mean- 
while been  diminishing,  and  at  length  the  spherical  error  pro- 
duced by  them  will  recover  its  original  proportion  to  the  op- 
posite error  of  the  curve  of  correction.  When  F  has  reached 
this  point  F"  (at  which  the  angle  of  incidence  does  not  exceed 
that  of  emergence  so  much  as  it  had  at  first  come  short  of  it), 
the  rays  again  pass  the  glass  free  from  spherical  aberration. 

"If  F  be  carried  from  hence  towards  the  glass,  or  outwards 
from  its  original  place,  the  angle  of  incidence  in  the  former 
case,  or  of  emergence  in  the  latter,  becomes  disproportionately 
effective,  and  either  way  the  aberration  exceeds  the  correction. 

"These  facts  have  been  established  by  careful  experiment: 
they  accord  with  every  appearance  in  such  combinations  of  the 
plano-convex  glasses  as  have  come  under  my  notice,  and  may, 
I  believe,  be  extended  to  this  rule,  that  in  general  an  achrom- 
atic object-glass,  of  which  the  inner  surfaces  are  in  contact,  or 
nearly  so,  will  have  on  one  side  of  it  two  foci  in  its  axis,  for  the 
rays  proceeding  from  whicli  it  will  be  truly  corrected  at  a 
moderate  aperture;  that  for  the  space  between  these  two  points 
its  spherical  aberration  will  be  over-corrected,  and  beyond 
them  either  way  under-corrected. 

"  The  longer  aplanatic  focus  may  be  found,  when  one  of  the 
plano-convex  object-glasses  is  placed  in  a  microscope,  by  short- 
ening the  tube,  if  the  glass  shows  over-correction;  if  under-cor- 
rection, by  lengthening  it,  or  by  bringing  the  rays  together, 
should  they  be  parallel  or  divergent,  by  a  very  small  good 
telescope.  The  shorter  focus  is  got  at  by  sliding  the  glass 
before  another  of  sufficient  length  and  large  aperture  that  is 
finely  corrected,  and  bringing  it  forwards  till  it  gives  the  re- 
flection of  a  bright  point  from  a  globule  of  quicksilver,  sharp 
and  free  from  mist,  when  the  distance  can  be  taken  between 
the  glass  and  the  object. 

*  *  The  longer  focus  is  the  place  at  which  to  ascertain  the 
utmost  aperture  that  may  be  given  to  the  glass,  and  where,  in 
the  absence  of  spherical  error,  its  exact  state  of  correction  as 
to  color  is  seen  most  distinctly. 

"  The  correction  of  the  chromatic  aberration,  like  that  of  the 
spherical,  tends  to  excess  in  the  marginal  rays;  so  that  if  a 


THE    MICROSCOPE.  27 

glass  which  is  achromatic,  with  a  moderate  aperture,  has  it 
cell  opened  wider,  the  circle  of  rays  thus  added  to  the  penci 
will  be  rather  over-corrected  as  to  color. 

"  The  same  tendency  to  over-correction  is  produced,  if,  with- 
out varying  the  aperture,  the  divergence  of  the  incident  rays  is 
much  augmented,  as  in  an  object-glass  placed  in  front  of  an- 
other; but  generally  in  this  position  a  part  only  of  its  aperture 
comes  into  use;  so  that  the  two  properties  mentioned  neutral- 
ize each  other,  and  its  chromatic  state  remains  unaltered.  If, 
for  example,  the  outstanding  colors  were  observed  at  the  longer 
focus  to  be  green  and  claret,  which  show  that  the  nearest  prac- 
ticable approach  is  made  to  the  union  of  the  spectrum,  they 
usually  continue  nearly  the  same  for  the  whole  space  between 
the  foci,  and  for  some  distance  beyond  them  either  way. 

"The  places  of  these  two  foci  and  their  proportions  to  each 
other  depend  on  a  variety  of  circumstances.  In  several  object- 
glasses  that  I  have  had  made  for  trial,  plano-convex,  with  their 
inner  surfaces  cemented,  their  diameters  the  radius  of  the  flint 
lens,  and  their  color  pretty  well  corrected,  those  composed  of 
dense  flint  and  light  plate  have  had  the  rays  from  the  longer 
focus  emerging  nearly  parallel;  and  this  focus  has  been  not 
quite  three  times  the  distance  of  the  shorter  from  the  glass: 
with  English  flint  the  rays  have  had  more  convergence,  and  the 
shorter  focus  has  borne  a  rather  less  proportion  to  the  longer. 

"If  the  surfaces  are  not  cemented,  a  striking  effect  is  pro- 
duced by  minute  differences  in  their  curves.  It  may  give  some 
idea  of  this,  that  in  a  glass  of  which  nearly  the  whole  disk  was 
covered  with  color  from  contact  of  the  lenses,  the  addition 
of  a  film  of  varnish,  so  thin  that  this  color  was  not  destroyed 
by  it,  caused  a  sensible  change  in  the  spherical  correction. 

"I  have  found  that  whatever  extended  the  longer  aplanatic 
focus,  and  increased  the  convergence  of  its  rays,  diminished 
the  relative  length  of  the  shorter.  Thus  by  turning  to  the  con- 
cave lens  the  flatter  instead  of  the  deeper  side  of  a  convex 
lens,  whose  radii  were  to  each  other  as  31  to  35,  the  pencil  of 
the  longer  aplanatic  focus,  from  being  greatly  divergent,  was 
brought  to  converge  at  a  very  small  distance  behind  the  glass; 
and  the  length  of  the  shorter  focus,  which  had  been  one-half 
that  of  the  longer,  became  but  one-sixth  of  it. 


28 


THE    MIOEOSOOPE. 


"The  direction  of  the  aplanatio  pencils  appears  to  be  scarcely 
affected  by  the  differences  in  the  thickness  of  glasses,  if  their 
state  as  to  color  is  the  same. 

"  One  other  property  of  the  double  object-glass  remains  to 
be  mentioned,  which  is,  that  when  the  longer  aplanatic  focus 
is  used,  the  marginal  rays  of  a  pencil  not  coincident  with  the 
axis  of  the  glass  are  distorted,  so  that  a  coma  is  thrown  out- 
wards; while  the  contrary  effect  of  a  coma  directed  towards  the 
centre  of  the  field  is  produced  by  the  rays  from  the  shorter 
focus.  These  peculiarities  of  the  coma  seem  inseparable  at- 
tendants on  the  two  foci,  and  are  as  conspicuous  in  the  achrom- 
atic meniscus  as  in  the  plano-convex  object-glass. 

"Of  several  purposes  to  which  the  particulars  just  given  seem 
applicable,  I  must  at  present  confine  myself  to  the  most  obvious 
one.  They  furnish  the  means  of  destroying  with  the  utmost 
ease  both  aberrations  in  a  large  focal  pencil,  and  of  thus  sur- 
mounting what  has  hitherto  been  the  chief  obstacle  to  the  per- 
fection of  the  microscope.  And  when  it  is  considered  that  the 
curves  of  its  diminutive  object-glasses  have  re- 
quired to  be  at  least  as  exactly  proportioned  as 
those  of  a  large  telescope  to  give  the  image  of  a 
bright  point  equally  sharp  and  colorless,  and 
that  any  change  made  to  correct  one  aberration 
was  liable  to  disturb  the  other,  some  idea  may 
be  formed  of  what  the  amount  of  that  obstacle 
must  have  been.  It  will,  however,  be  evident 
that  if  any  object-glass  is  but  made  achromatic, 
with  its  lenses  truly  worked  and  cemented,  so 
that  their  axes  coincide,  it  may  with  certainty 
be  connected  with  another  possessing  the  same 
requisites  and  of  suitable  focus,  so  that  the 
combination  shall  be  free  from  spherical  error 
also  in  the  centre  of  its  field.  For  this  the 
rays  have  only  to  be  received  by  the  front  glass 
B  (Fig.  14)  from  its  shorter  aplanatic  focus  F", 
and  transmitted  in  the  direction  of  the  longer 
correct  pencil  F  A  of  the  other  glass  A.  It  is 
desirable  that  the  latter  pencil  should  neither 
Fig.  14.  converge  to  a  very  short  focus  nor  be  more  than 


THE    MIOBOSOOPB.  %\) 

very  slightly  if  at  all  divergent;  and  a  little  attention  at  first  to 
the  kind  of  glass  used  will  keep  it  within  this  range,  the  denser 
flint  being  suited  to  the  glasses  of  shorter  focus  and  larger  angle 
of  aperture. 

"The  adjustment  of  the  microscope  is  then  perfected,  if 
necessary,  by  slightly  varying  the  distance  between  the  object- 
glasses;  and  after  that  is  done,  the  length  of  the  tube  which 
carries  the  eye-pieces  may  be  altered  greatly  without  disturbing 
the  correction,  opposite  errors  which  balance  each  other  being 
produced  by  the  change. 

"If  the  two  glasses  which  in  the  diagram  are  drawn  at  some 
distance  apart  are  brought  nearer  together  (if  the  place  of  A, 
for  instance,  is  carried  to  the  dotted  figure),  the  rays  trans- 
mitted by  B  in  the  direction  of  the  longer  aplanatic  pencil  of  A 
will  plainly  be  derived  from  some  point  Z  more  distant  than 
F",  and  lying  between  the  aplanatic  foci  of  B;  therefore  (ac- 
cording to  what  has  been  stated)  this  glass,  and  consequently 
the  combination,  will  then  be  spherically  over-corrected.  If, 
on  the  other  hand,  the  distance  between  A  and  B  is  increased, 
the  opposite  effects  are  of  course  produced. 

"  In  combining  several  glasses  together  it  is  often  convenient 
to  transmit  an  under-corrected  pencil  from  the  front  glass,  and 
to  counteract  its  error  by  over-correction  in  the  middle  one. 

"  Slight  errors  in  color  may  in  the  same  manner  be  destroyed 
by  opposite  ones;  and  on  the  principles  described  we  not  only 
acquire  fine  correction  for  the  central  ray,  but  by  the  opposite 
effects  at  the  two  foci  on  the  transverse  pencil,  all  coma  can  be 
destroyed,  and  the  whole  field  rendered  beautifully  flat  and 
distinct. " 

Mr.  Lister's  paper  enters  into  further  particulars,  which  are 
not  essential  to  the  comprehension  of  the  subject.  It  is  suf- 
ficient to  say  that  his  investigations  and  results  proved  to  be 
of  the  highest  value  to  the  practical  optician,  and  the  progress 
of  improvement  was  in  consequence  extremely  rapid.  The  new 
principles  were  applied  and  exhibited  by  Mr.  Hugh  Powell  and 
Mr.  Andrew  Boss  with  a  degree  of  success  which  had  never 
been  anticipated;  so  perfect  indeed  were  the  corrections  given 
to  the  achromatic  object-glass — so  completely  were  the  errors 
of  sphericity  and  dispersion  balanced  or  destroyed — that  the 


30  THE    MICKOSOOPE. 

circumstance  of  covering  the  object  with  a  plate  of  the  thin- 
nest glass  or  talc  disturbed  the  corrections,  if  they  had  been 
adapted  to  an  uncovered  object,  and  rendered  an  object-glass 
which  was  perfect  under  one  condition  sensibly  defective  under 
the  other. 

This  defect,  if  that  should  be  called  a  defect  which  arose  out 
of  improvement,  was  first  discovered  by  Mr.  Boss,  who  imme- 
diately suggested  the  means  of  correcting  it,  and  presented  to 
the  Society  of  Arts,  in  1837,  a  paper  on  the  subject,  which  was 
published  in  the  51st  volume  of  their  Transactions,  and  which, 
as  it  is,  like  Mr.  Lister's  essential  to  a  full  understanding  of  the 
ultimate  refinements  of  the  instrument,  we  shall  extract  nearly 
in  full: 

* '  In  the  course  of  a  practical  investigation  (says  Mr.  Boss) 
with  the  view  of  constructing  a  combination  of  lenses  for  the 
object-glass  of  a  compound  microscope,  which  should  be  free 
from  the  effects  of  aberration,  both  for  central  and  oblique  pen- 
cils of  great  angle,  I  combined  the  condition  of  the  greatest 
possible  distance  between  the  object  and  object-glass;  for  in 
object-glasses  of  short  focal  length  their  closeness  to  the  object 
has  been  an  obstacle  in  many  cases  to  the  use  of  high  magnify- 
ing powers,  and  is  a  constant  source  of  inconvenience. 

"In  the  improved  combination,  the  diameter  is  only  suf- 
ficient to  admit  the  proper  pencil;  the  convex  lenses  are 
wrought  to  an  edge,  and  the  concave  have  only  sufficient  thick- 
ness to  support  their  figure;  consequently  the  combination  is 
the  thinnest  possible,  and  it  follows  that  there  will  be  the  great- 
est distance  between  the  object  and  the  object-glass.  The  focal 
length  is  one-eighth  of  an  inch,  having  an  angular  aperture  of 
60°,  with  a  distance  of  l-25th  of  an  inch,  and  a  magnifying 
power  of  970  times  linear,  with  perfect  definition  on  the  most 
difficult  Podura  scales.  I  have  made  object-glasses  l-16th  of 
an  inch  focal  length;  but  as  the  angular  aperture  cannot  be  ad- 
vantageously increased,  if  the  greatest  distance  between  the 
object  and  object-glass  is  preserved,  their  use  will  be  very  lim- 
ited. 

"The  quality  of  the  definition  produced  by  an  achromatic 
compound  microscope  will  depend  upon  the  accuracy  with 
which  the  aberrations,  both  chromatic  and  spherical,  are  bal- 


THE    MICROSCOPE. 


31 


anced,  together  with  the  general  perfection  of  the  workman- 
ship. Now,  in  Wollaston's  doublets,  and  Holland's  triplets, 
there  are  no  means  of  producing  a  balance  of  the  aberrations, 
as  they  are  composed  of  convex  lenses  only;  therefore  the  best 
that  can  be  done  is  to  make  the  aberrations  a  minimum;  the 
remaining  positive  aberration  in  these  forms  produces  its  pecu- 
liar effect  upon  objects  (particularly  the  detail  of  the  thin 
transparent  class),  which  may  lead  to  misapprehension  of  their 
true  structure;  but  with  the  achromatic  object-glass,  where  the 
aberrations  are  correctly  balanced,  the  most  minute  parts  of  an 
object  are  accurately  displayed,  so  that  a  satisfactory  judgment 
of  their  character  may  be  formed. 

"  It  will  be  seen  by  Fig.  15,  that  when  a  certain  angular  pen- 
cil A  O  A'  proceeds  from  the  object  O,  and  is  incident  on  the 
plane  side  of  the  first  lens,  if  the  combination  is  removed  from 


V 


Fig.  15. 


the  object,  as  in  Fig.  16,  the  extreme  rays  of  the  pencil  im- 
pinge on  the  more  marginal  parts  of  the  glass,  and  as  the  re- 
fractions are  greater  here,  the  aberrations  will  be  greater  also. 
Now,  if  two  compound  object-glasses  have  their  aberrations 
balanced,  one  being  situated  as  in  Fig.  15,  and  the  other  as  in 
Fig.  16,  and  the  same  disturbing  power  applied  to  both,  that 
in  which  the  angles  of  incidence  and  the  aberrations  are  small 
will  not  be  so  much  disturbed  as  where  the  angles  are  great, 
and  where  consequently  the  aberrations  increase  rapidly. 


32 


THE    MICBOSCOPE. 


"  When  an  object-glass  has  its  aberrations  balanced  for  view- 
ing an  opaque  object,  and  it  is  required  to  examine  that  object 
by  transmitted  light,  the  correction  will  remain;  but  if  it  is 
necessary  to  immerse  the  object  in  a  fluid,  or  to  cover  it  with 
glass  or  talc,  an  aberration  will  arise  from  these  circumstances, 
which  will  disturb  the  previous  correction,  and  consequently 
deteriorate  the  definition;  and  this  effect  will  be  more  obvious 
with  the  increase  of  the  distance  between  the  object  and  the 
object-glass. 

IP  /E  E, 

R> 


Fig.  17. 

"The  aberration  produced  with  diverging  rays  by  a  piece  of 
flat  and  parallel  glass,  such  as  would  be  used  -J or  covering  an 
object,  is  represented  at  Fig.  17,  where  G  G  G  G  is  the  refract- 
ing medium,  or  piece  of  glass  covering  the  object  O;  O  P,  the 
axis  of  the  pencil,  perpendicular  to  the  flat  surfaces;  O  T,  a 
ray  near  the  axis;  and  O  T',  the  extreme  ray  of  the  pencil  inci- 
dent on  the  under  surface  of  the  glass;  then  T  E,  T'  E',  will  be 
the  directions  of  the  rays  in  the  medium,  and  E  E,  E'  E',  those 
of  the  emergent  rays.  Now  if  the  course  of  these  rays  is  con- 
tinued, as  by  the  dotted  lines,  they  will  be  found  to  intersect 
the  axis  at  different  distances,  X  and  Y,  from  the  surface  of  the 
glass;  and  the  distance  X  Y  is  the  aberration  produced  by  the 
medium  which,  as  before  stated,  interferes  with  the  previously 
balanced  aberrations  of  the  several  lenses  composing  the  object- 


THE    MICROSCOPE.  33 

glass.  There  are  many  cases  of  this,  but  the  one  here  selected 
serves  best  to  illustrate  the  principle.  I  need  not  encumber 
the  description  with  the  theoretical  determination  of  this  quan- 
tity, as  it  varies  with  exceedingly  minute  circumstances  which 
we  cannot  accurately  control;  such  as  the  distance  of  the  object 
from  the  under  side  of  the  glass,  and  the  slightest  difference  in 
the  thickness  of  the  glass  itself;  and  if  these  data  could  be 
readily  obtained,  the  knowledge  would  be  of  no  utility  in 
making  the  correction,  that  being  wholly  of  a  practical  nature. 

"If  an  object-glass  is  constructed  as  represented  in  Fig.  16, 
where  the  posterior  combination  P  and  the  middle  M  have 
together  an  excess  of  negative  aberration,  and  if  this  be  cor- 
rected by  the  anterior  combination  A,  having  an  excess  of  posi- 
tive aberration,  then  this  latter  combination  can  be  made  to  act 
more  or  less  powerfully  upon  P  and  M,  by  making  it  approach 
to  or  recede  from  them;  for  when  the  three  are  in  close  con- 
tact, the  distance  of  the  object  from  the  object-glass  is  greatest; 
and  consequently  the  rays  from  the  object  are  diverging  from  a 
point  at  a  greater  distance  than  when  the  combinations  are 
separated;  and  as  a  lens  bends  the  rays  more,  or  acts  with 
greater  effect,  the  more  distant  the  object  is  from  which  the 
rays  diverge,  the  effect  of  the  anterior  combination  A  upon  the 
other  two,  P  and  M,  will  vary  with  its  distance  from  thence. 
When  therefore  the  correction  of  the  whole  is  effected  for  an 
opaque  object  with  a  certain  distance  between  the  anterior  and 
middle  combination,  if  they  are  then  put  in  contact,  the  dis- 
tance between  the  object  and  object-glass  will  be  increased; 
consequently  the  anterior  combination  will  act  more  power- 
fully, and  the  whole  will  have  an  excess  of  positive  aberration. 
Now  the  effect  of  the  aberration  produced  by  a  piece  of  flat  and 
parallel  glass  being  of  the  negative  character,  it  is  obvious  that 
the  above  considerations  suggest  the  means  of  correction  by 
moving  the  lenses  nearer  together,  till  the  positive  aberration 
thereby  produced  balances  the  negative  aberration  caused  by 
the  medium. 

"The  preceding  refers  only  to  the  spherical  aberration,  but 
the  effect  of  the  chromatic  is  also  seen  when  an  object  is  cov- 
ered with  a  piece  of  glass;  for,  in  the  course  of  my  experiments, 
I  observed  that  it  produced  a  chromatic  thickening  of  the  out- 


34  THE    MICROSCOPE. 

line  of  the  Podura  and  other  delicate  scales;  and  if  diverging 
rays  near  the  axis  and  at  the  margin  are  projected  through  a 
piece  of  flat  parallel  glass,  with  the  various  indices  of  refraction 
for  the  different  colors,  it  will  be  seen  that  each  ray  will  emerge 
separated  into  a  beam  consisting  of  the  component  colors  of 
the  ray,  and  that  each  beam  is  widely  different  in  form.  This 
difference,  being  magnified  by  the  power  of  the  microscope, 
readily  accounts  for  the  chromatic  thickening  of  the  outline 
just  mentioned.  Therefore  to  obtain  the  finest  definition  of 
extremely  delicate  and  minute  objects,  they  should  be  viewed 
without  a  covering;  if  it  be  desirable  to  immerse  them  in  a  fluid, 
they  should  be  covered  with  the  thinnest  possible  film  of  talc, 
as,  from  the  character  of  the  chromatic  aberration,  it  will  be 
seen  that  varying  the  distances  of  the  combinations  will  not 
sensibly  affect  the  correction;  though  object-lenses  may  be 
made  to  include  a  given  fluid  or  solid  medium  in  their  cor- 
rection for  color. 

"The  mechanism  for  applying  these  principles  to  the  cor- 
rection of  an  object-glass  under  the  various  circumstances,  is 
represented  in  Fig.  18,  where  the  anterior  lens  is  set  in  the  end 


Fig.  18. 


THE    MIOBOSCOPE.  35 

of  a  tube  A  A,  which  slides  on  the  cylinder  B  containing  the  re- 
mainder of  the  combination;  the  tube  A  A,  holding  the  lens 
nearest  the  object,  may  then  be  moved  upon  the  cylinder  B, 
for  the  purpose  of  varying  the  distance  according  to  the  thick- 
ness of  the  glass  covering  the  object,  by  turning  the  screwed 
ring  C  0,  or  more  simply  by  sliding  the  one  on  the  other,  and 
clamping  them  together  when  adjusted.  An  aperture  is  made 
in  the  tube  A,  within  which  is  seen  a  mark  engraved  on  the 
cylinder,  and  on  the  edge  of  which  are  two  marks,  a  longer 
and  a  shorter,  engraved  upon  the  tube.  When  the  mark  on 
the  cylinder  coincides  with  the  longer  mark  on  the  tube,  the 
adjustment  is  perfect  for  an  uncovered  object;  and  when  the 
coincidence  is  with  the  short  mark,  the  proper  distance  is  ob- 
tained to  balance  the  aberrations  produced  by  glass  one- 
hundredth  of  an  inch  thick,  and  such  glass  can  be  readily  sup- 
plied. 

"It  is  hardly  necessary  to  observe,  that  the  necessity  for  this 
correction  is  wholly  independent  of  any  particular  construction 
of  the  object-glass;  as  in  all  cases  where  the  object-glass  is  cor- 
rected for  an  object  uncovered,  any  covering  of  glass  will  create 
a  different  value  of  aberration  to  the  first  lens,  which  previ- 
ously balanced  the  aberration  resulting  from  the  rest  of  the 
lenses;  and  as  this  disturbance  is  effected  at  the  first  refraction, 
it  is  independent  of  the  other  part  of  the  combination.  The 
visibility  of  the  effect  depends  on  the  distance  of  the  object 
from  the  object-glass,  the  angle  of  the  pencil  transmitted,  the 
focal  length  of  the  combination,  the  thickness  of  the  glass 
covering  the  object,  and  the  general  perfection  of  the  correc- 
tions for  chromatism  and  the  oblique  pencils. 

"With  this  adjusting  object-glass,  therefore,  we  can  have 
the  requisites  of  the  greatest  possible  distance  between  the  ob- 
ject and  object-glass,  an  intense  and  sharply  defined  image 
throughout  the  field  from  the  large  pencil  transmitted,  and  the 
accurate  correction  of  the  aberrations;  also,  by  the  adjustment, 
the  means  of  preserving  that  correction  under  all  the  varied 
circumstances  in  which  it  may  be  necessary  to  place  an  object 
for  the  purpose  of  observation." 

In  the  annexed  engraving,  Fig.  19,  we  have  shown  the  triple 
achromatic  object-glass  in  connection  with  the  eye-piece  con- 


36 


THE  MICROSCOPE. 


sisting  of  the  field-glass  F  F,  and 
the  eye-glass  E  E,  forming  together 
the  modern  achromatic  microscope. 
The  course  of  the  light  is  shown  by 
drawing  three  rays  from  the  centre 
and  three  from  each  end  of  the  ob- 
ject O.  These  rays  would,  if  left  to 
themselves,  form  an  image  of  the 
object  at  A  A,  but  being  bent  and 
converged  by  the  field-glass  F  F, 
they  form  the  image  at  B  B,  where  a 
stop  is  placed  to  intercept  all  light 
except  what  is  required  for  the  for- 
mation of  the  image.  From  B  B 
therefore  the  rays  proceed  to  the 
eye-glass  exactly  as  has  been  de- 
scribed in  reference  to  the  simple 
microscope  and  to  the  compound  of 
two  glasses. 

If  we  stopped  here  we  should  con- 
vey a  very  imperfect  idea  of  the 
beautiful  series  of  corrections  effect- 
ed by  the  eye-piece,  and  which  were 
first  pointed  out  in  detail  in  a  paper 
on  the  subject  published  by  Mr.  Var- 
ley  in  the  51st  volume  of  the  Tran- 
sactions of  the  Society  of  Arts.  The 
eye-piece  in  question  was  invented 
by  Huyghens  for  telescopes,  with  no 
other  view  than  that  of  diminishing 
the  spherical  aberration  by  producing 
the  refractions  at  two  glasses  instead 
of  one,  and  of  increasing  the  field 
of  view.  It  was  reserved  for  Bosco- 
vich  to  point  out  that  Huyghens 
had  by  this  arrangement  accidentally 
corrected  a  great  part  of  the  chrom- 
atic aberration,  and  this  subject  is 
further  investigated  with  much  skill 


u..., 


THE    MICKOSCOPE. 


37 


in  two  papers  by  Professor  Airy  in  the  Cambridge  Philosophical 
Transactions,  to  which  we  refer  the  mathematical  reader.  These 
investigations  apply  chiefly  to  the  telescope,  where  the  small 
pencils  of  light  and  great  distance  of  the  object  exclude  con- 
siderations which  become  important  in  the  microscope,  and 
which  are  well  pointed  out  in  Mr.  Varley's  paper  before  men- 
tioned. 


Fig.  20. 

Let  Fig.  20  represent  the  Huyghenean  eye-piece  of  a  micro- 
scope; F  F  and  E  E  being  the  field-glass  and  eye-glass,  and 
L  M  N  the  two  extreme  rays  of  each  of  the  three  pencils,  eman- 
ating from  the  centre  and  ends  of  the  object,  of  which,  but  for 
the  field-glass,  a  series  of  colored  images  would  be  formed  from 
K  II  to  B  B;  those  near  B  B  being  red,  those  near  B  B  blue, 
and  the  intermediate  ones  green,  yellow,  and  so  on,  correspond- 
ing with  the  colors  of  the  prismatic  spectrum,  This  order  of 


38  THE    MICEOSCOPE. 

colors,  it  will  be  observed,  is  the  reverse  of  that  described  in 
treating  of  the  common  compound  microscope  (Fig.  12),  in 
which  the  single  object-glass  projected  the  red  image  beyond 
the  blue.  The  effect  just  described,  of  projecting  the  blue  im- 
age beyond  the  red,  is  purposely  produced  for  reasons  presently 
to  be  given,  and  is  called  over-correcting  the  object-glass  as  to 
color.  It  is  to  be  observed  also  that  the  images  B  B  and  R  R 
are  curved  in  the  wrong  direction  to  be  distinctly  seen  by  a 
convex  eye-lens,  and  this  is  a  further  defect  of  the  compound 
microscope  of  two  lenses.  But  the  field-glass,  at  the  same  time 
that  it  bends  the  rays  and  converges  them  to  foci  at  B'  B'  and 
R'  R',  also  reverses  the  curvature  of  the  images  as  there  shown, 
and  gives  them  the  form  best  adapted  for  distinct  vision  by  the 
eye-glass  E  E.  The  field-glass  has  at  the  same  time  brought 
the  blue  and  red  images  closer  together,  so  that  they  are 
adapted  to  pass  uncolored  through  the  eye-glass.  To  render 
this  important  point  more  intelligible,  let  it  be  supposed  that 
the  object-glass  had  not  been  over-corrected,  that  it  had  been 
perfectly  achromatic;  the  rays  would  then  have  become  colored 
as  soon  as  they  had  passed  the  field-glass;  the  blue  rays,  to  take 
the  central  pencil,  for  example,  would  converge  at  b  and  the  red 
rays  at  r,  which  is  just  the  reverse  of  what  the  eye-lens  re- 
quires; for  as  its  blue  focus  is  also  shorter  than  its  red,  it  would 
demand  rather  that  the  blue  image  should  be  at  r  and  the  red 
at  b.  This  effect  we  have  shown  to  be  produced  by  the  over- 
correction  of  the  object-glass,  which  protrudes  the  blue  foci 
B  B  as  much  beyond  the  red  foci  R  R  as  the  jsum  of  the  dis- 
tances between  the  red  and  blue  foci  of  the  field-lens  and  eye- 
lens;  so  that  the  separation  B  R  is  exactly  taken  up  in  passing 
through  those  two  lenses,  and  the  whole  of  the  colors  coincide 
as  to  focal  distance  as  soon  as  the  rays  have  passed  the  eye-lens. 
But  while  they  coincide  as  to  distance,  they  differ  in  another 
respect;  the,  blue  images  are  rendered  smaller  than  the  red  by 
the  superior  refractive  power  of  the  field-glass  upon  the  blue 
rays.  In  tracing  the  pencil  L,  for  instance,  it  will  be  noticed 
that  after  passing  the  field-glass,  two  sets  of  lines  are  drawn, 
one  whole,  and  one  dotted,  the  former  representing  the  red, 
and  the  latter  the  blue  rays.  This  is  the  accidental  effect  in 
the  Huyghenean  eye-piece  pointed  out  by  Boscovich.  This 


THE    MICBOSOOPB.  6\) 

separation  into  colors  at  the  field-glass  is  like  the  over-correc- 
tion of  the  object-glass;  it  leads  to  a  subsequent  complete  cor- 
rection. For  if  the  differently  colored  rays  were  kept  together 
till  they  reached  the  eye-glass,  they  would  then  become  col- 
ored, and  present  colored  images  to  the  eye;  but  fortunately, 
and  most  beautifully,  the  separation  effected  by  the  field-glass 
causes  the  blue  rays  to  fall  so  much  nearer  the  centre  of  the 
eye-glass,  where,  owing  to  the  spherical  figure,  the  refractive 
power  is  less  than  at  the  margin,  that  the  spherical  error  of  the 
eye-lens  constitutes  a  nearly  perfect  balance  to  the  chromatic 
dispersion  of  the  field-lens,  and  the  red  and  blue  rays  L'  and 
L"  emerge  sensibly  parallel,  presenting,  in  consequence,  the 
perfect  definition  of  a  single  point  to  the  eye.  The  same  rea- 
soning is  true  of  the  intermediate  colors  and  of  the  other  pen- 
cils. 

From  what  has  been  stated  it  is  obvious  that  we  mean  by  an 
achromatic  object-glass  one  in  which  the  usual  order  of  dis- 
persion is  so  far  reversed  that  the  light,  after  undergoing  the 
singularly  beautiful  series  of  changes  effected  by  the  eye-piece, 
shall  come  uncolored  to  the  eye.  We  can  give  no  specific  rules 
for  producing  these  results.  Close  study  of  the  formulae  for 
achromatism  given  by  the  celebrated  mathematicians  we  have 
quoted  will  do  much,  but  the  principles  must  be  brought  to  the 
test  of  repeated  experiment.  Nor  will  the  experiments  be  worth 
anything,  unless  the  curves  be  most  accurately  measured  and 
worked,  and  the  lenses  centered  and  adjusted  with  a  degree  of 
precision  which,  to  those  who  are  familiar  only  with  telescopes, 
will  be  quite  unprecedented. 

The  Huyghenean  eye-piece  which  we  have  described  is  the 
best  for  merely  optical  purposes,  but  when  it  is  required  to 
measure  the  magnified  image,  we  use  the  eye-piece  invented  by 
Mr.  Bamsden,  and  called,  from  its  purpose,  the  micrometer 
eye-piece.  When  it  is  stated  that  we  sometimes  require  to 
measure  portions  of  animal  or  vegetable  matter  a  hundred  times 
smaller  than  any  divisions  that  can  be  artificially  made  on  any 
measuring  instrument,  the  advantage  of  applying  the  scale  to 
the  magnified  image  will  be  obvious,  as  compared  with  the  ap- 
plication of  engraved  or  mechanical  micrometers  to  the  stage 
of  the  instrument. 


THE    MIOBOSCOPE. 


The  arrangement  is  shown  in  Fig.  21,  where  E  E  and  F  F 
are  the  eye  and  field  glass,  the  latter  having  now  its  plane  face 
towards  the  object.  The  rays  from  the  object  are  here  made  to 
converge  at  A  A,  immediately  in  front  of  the  field-glass,  and 
here  also  is  placed  a  plane  glass  on  which  are  engraved  divisions 
of  a  hundredth  of  an  inch  or  less. 
The  markings  of  these  divisions  come 
into  focus  therefore  at  the  same  time 
as  the  image  of  the  object,  and  both 
are  distinctly  seen  together.  Thus 
the  measure  of  the  magnified  image 
is  given  by  mere  inspection,  and  the 
value  of  such  measures  in  reference 
to  the  real  object  may  be  obtained 
thus,  which,  when  once  obtained,  is 
constant  for  the  same  object-glass. 
Place  on  the  stage  of  the  instrument 
a  divided  scale  the  value  of  which  is 
known,  and  viewing  this  scale  as  the 
microscopic  object,  observe  how  many 
of  the  divisions  on  the  scale  attached 
to  the  eye-piece  correspond  with  one 
of  those  in  the  magnified  image.  If, 
for  instance,  ten  of  those  in  the  eye- 
piece correspond  with  one  of  those  in 
the  image,  and  if  the  divisions  are 

known  to  be  equal,  then  the  image  is  ten  times  larger  than  the 
object,  and  the  dimensions  of  the  object  are  ten  times  less 
than  indicated  by  the  micrometer.  If  the  divisions  on  the 
micrometer  and  on  the  magnified  scale  were  not  equal,  it  be- 
comes a  mere  rule-of-three  sum,  but  in  general  this  trouble  is 
taken  by  the  maker  of  the  instrument,  who  furnishes  a  table 
showing  the  value  of  each  division  of  the  micrometer  for  every 
object-glass  with  which  it  may  be  used. 

While  on  the  subject  of  measuring  it  may  be  well  to  explain 
the  mode  of  ascertaining  the  magnifying  power  of  the  com- 
pound microscope,  which  is  generally  taken  on  the  assumption 
before  mentioned,  that  the  naked  eye  sees  most  distinctly  at 
the  distance  of  ten  inches. 


\ 

3 

Fig.  21. 


THE    MICROSCOPE.  41 

Place  on  the  stage  of  the  instrument,  as  before,  a  known 
divided  scale,  and  when  it  is  distinctly  seen,  hold  a  rule  at  ten 
inches  distance  from  the  disengaged  eye,  so  that  it  may  be  seen 
by  that  eye,  overlapping  or  lying  by  side  of  the  magnified  pic- 
ture of  the  other  scale.  Then  move  the  rule  till  one  or  more 
of  its  known  divisions  correspond  with  a  number  of  those  in 
the  magnified  scale,  and  a  comparison  of  the  two  gives  the 
magnifying  power. 

Having  now  explained  the  optical  principles  of  the  achromatic 
compound  microscope,  it  remains  only  to  describe  the  mechan- 
ical arrangements  for  giving  those  principles  their  full  effect. 
The  mechanism  of  a  microscope  is  of  much  more  importance 
than  might  be  imagined  by  those  who  have  not  studied  the  sub- 
ject. In  the  first  place,  steadiness,  or  freedom  from  vibration, 
and  most  particularly  freedom  from  any  vibrations  which  are  not 
equally  communicated  to  the  object  under  examination,  and 
to  the  lenses  by  which  it  is  viewed,  is  a  point  of  the  ut- 
most consequence.  When,  for  instance,  the  body  contain- 
ing the  lenses  is  screwed  by  its  lower  extremity  to  a  horizon- 
tal arm,  we  have  one  of  the  most  vibratory  forms  conceivable; 
it  is  precisely  the  form  of  the  inverted  pendulum,  which  is  ex- 
pressly contrived  to  indicate  otherwise  insensible  vibrations. 
The  tremor  necessarily  attendant  on  such  an  arrangement 
is  magnified  by  the  whole  power  of  the  instrument;  and  as 
the  object  on  the  stage  partakes  of  this  tremor  in  a  com- 
paratively insensible  degree,  the  image  is  seen  to  oscillate 
so  rapidly,  as  in  some  cases  to  be  wholly  undistinguishable. 
Such  microscopes  cannot  possibly  be  used  with  high  pow- 
ers in  ordinary  houses  abutting  on  any  paved  streets  through 
which  carriages  are  passing,  nor  indeed  are  they  adapted  to 
be  used  in  houses  in  which  the  ordinary  internal  sources  of 
shaking  exist. 

One  of  the  best  modes  of  mounting  a  compound  microscope 
is  shown  in  the  annexed  view  (Fig.  22),  which,  though  too  min- 
ute to  exhibit  all  the  details,  will  serve  to  explain  the  chief  fea- 
tures of  the  arrangement. 

A  massy  pillar  A  is  screwed  into  a  solid  tripod  B,  and  is  sur- 
mounted by  a  strong  joint  at  C,  on  which  the  whole  instru- 
ment turns,  so  as  to  enable  it  to  take  a  perfectly  horizontal  or 


42  THE    MICKOSOOPE. 

vertical  position,  or  any  intermediate  angle,  such,  for  instance, 
as  that  shown  in  the  engraving. 

This  movable  portion  of  the  instrument  consists  of  one  solid 
casting  D  E  F  G;  from  F  to  G  being  a  thick  pierced  plate  car- 
rying the  stage  and  its  appendages.  The  compound  body  H  is 
attached  to  the  bar  D  E,  and  moves  up  and  down  upon  it  by  a 
rack  and  pinion  worked  by  either  of  the  milled  heads  K.  The 
piece  D  E  F  G  is  attached  to  the  pillar  by  the  joint  C,  which 
being  the  source  of  the  required  movement  in  the  instrument, 
is  obviously  its  weakest  part,  and  about  which  no  doubt  consid- 
erable vibration  takes  place.  But  inasmuch  as  the  piece 
D  E  F  G  of  necessity  transmits  such  vibrations  equally  to  the 
body  of  the  microscope  and  to  the  objects  on  the  stage,  they 
hold  always  the  same  relative  position,  and  no  visible  vibration 
is  caused,  how  much  soever  may  really  exist.  To  the  under 
side  of  the  stage  is  attached  a  circular  stem  L,  on  which  slides 
the  mirror  M,  plane  on  one  side  and  concave  on  the  other,  to 
reflect  the  light  through  the  aperture  in  the  stage.  Beneath 
the  stage  is  a  circular  revolving  plate  containing  three  apertures 
of  various  sizes,  to  limit  the  angle  of  the  pencil  of  light  which 
shall  be  allowed  to  fall  on  the  object  under  examination.  Be- 
sides these  conveniences  the  stage  has  a  double  movement  pro- 
duced by  two  racks  at  right  angles  to  each  other,  and  worked  by 
milled  heads  beneath.  It  has  also  the  usual  appendages  of 
forceps  to  hold  minute  objects,  and  a  lens  to  condense  the 
light  upon  them,  all  of  which  are  well  understood,  and  if  not, 
will  be  rendered  more  intelligible  by  a  few  minutes'  examina- 
tion of  a  microscope  than  by  the  most  lengthened  description. 
One  other  point  remains  to  be  noticed.  The  movement  pro- 
duced by  the  milled  head  K  is  not  sufficiently  delicate  to  adjust 
the  focus  of  very  powerful  lenses,  nor  indeed  is  any  rack  move- 
ment. Only  the  finest  screws  are  adapted  to  this  purpose; 
and  even  these  are  improved  by  means  for  reducing  the  rapid- 
ity of  the  screw's  movement.  For  this  purpose  the  lower  end 
of  the  compound  body  H,  which  carries  the  object-glass, 
consists  of  a  piece  of  smaller  tube  sliding  in  parallel  guides 
in  the  main  body,  and  kept  constantly  pressed  upwards  by 
a  spiral  spring  but  it  can  be  drawn  downward  by  a  lever  cross- 
ing the  body,  and  acted  on  by  an  extremely  fine  screw 


THE    MICROSCOPE. 


43 


Fig.  22. 


44  THE    MIOKOSOOPE. 

whose  milled  head  is  seen  at  N,  and  the  fineness  of  which 
is  tripled  by  means  of  the  lever  through  which  it  acts  on  the 
object-glass.  The  instrument  is  of  course  roughly  adjusted  by 
the  rack  movement,  and  finished  by  the  screw,  or  by  such 
other  means  as  are  chosen  for  the  purpose.  One  very  inge- 
nious contrivance,  but  applied  to  the  stage,  instead  of  the 
body  of  the  microscope,  invented  by  Mr.  Powell,  will  be  found 
described  in  the  50th  volume  of  the  Transactions  of  the  So- 
ciety of  Arts. 

The  greater  part  of  the  directions  for  viewing  and  illumin- 
ating objects  given  in  reference  to  the  simple  microscope  are 
applicable  to  the  compound.  An  argand  lamp  placed  in  the 
focus  of  a  large  detached  lens  so  as  to  throw  parallel  rays  upon 
the  mirror,  is  the  best  artificial  light;  and  for  opaque  objects 
the  light  so  thrown  up  may  be  reflected  by  metallic  specula 
(called,  from  their  inventor,  Lieberkhuns)  attached  to  the  ob- 
ject-glasses. 

It  has  been  recently  proposed  by  Sir  David  Brewster  and  by 
M.  Dujardin  to  render  the  Wollaston  condenser  achromatic, 
and  they  have  accordingly  been  made  with  three  pairs  of 
achromatic  lenses  instead  of  the  single  lens  before  described, 
with  very  excellent  effect.  The  last-mentioned  gentleman  has 
also  projected  an  ingenious  apparatus,  called  the  Hyptioscope, 
attached  to  the  eye-piece  for  the  purpose  of  erecting  the  mag- 
nified picture. 

The  erector  commonly  applied  to  the  compound  microscope 
consists  of  a  pair  of  lenses  acting  like  the  erecting  eye-piece  of 
the  telescope.  But  this,  though  it  is  convenient  for  the  pur- 
pose of  dissection,  very  much  impairs  the  op- 
tical performance  of  the  instrument. 

For  drawing  the  images  presented  by  the 
microscope  the  best  apparatus  consists  of  a 
mirror  M  (Fig.  23),  composed  of  a  thin  piece 
of  rather  dark-colored  glass  cemented  on  to  a 
piece  of  plate-glass  inclined  at  an  angle  of  45° 
in  front  of  the  eye-glass  E.  The  light  es- 
caping from  the  eye-glass  is  assisted  in  its  re- 
flection upwards  to  the  eye  by  the  dark  glass,  j?ig.  23. 
which  effects  the  further  useful  purpose  of 


THE    MICROSCOPE. 


45 


rendering  the  paper  less  brilliant,  and  thus  enabling  the  eye 
better  to  see  the  reflected  image.  The  lens  L  below  the  reflector 
is  to  cause  the  light  from  the  paper  and  pencil 
to  diverge  from  the  same  distance  as  that  re- 
ceived from  the  eye-glass;  in  other  words,  to 
cause  it  to  reach  the  eye  in  parallel  lines. 

Dr.  Wollaston's  camera  lucida,  as  shown  in 
Fig.  24,  is  sometimes  attached  to  the  eye-piece 
of  the  microscope  for  the  same  purpose.     In 
this  instrument  the  rays  suffer  two  internal  re- 
flections within  the  glass  prism,  as  will  be  seen 
explained  in  the  article  "  Camera  Lucida."   In 
this  minute  figure  we  have  omitted  to  trace  the  reflected  rays, 
merely  to  avoid  confusion. 

Annexed  are  four  engravings  of  microscopic  objects,  the  true 
character  of  which  it  is,  however,  impossible  to  give  in  wood,  and 
is  difficult  indeed  to  accomplish  by  any  description  of  engraving. 


Fig. 


Fig.  25. 


Fig.  26. 


Fig.  27. 


THE    MICROSCOPE. 


Fig.   25  shows  a  scale  of  the  small  insect  called  Podura 
Plumbea,  the  common  Skiptail,  magnified  about  five  hundred 
times.     To  define  the  markings  on  this  scale  clearly  is  the  high- 
est test  of  a  deep  achromatic  object-glass;  and  this  drawing  is 
given  rather  to  explain  what  the  observer  should 
look  for,  than  as  a  very  correct  representation. 
Fig.  26  is  a  scale  or  feather  of  the  Menelaus  But- 
terfly; Fig.  27  is  the  hair  of  a  singular  insect, 
the  Dermestes;  and  Fig.  28  is  a  longitudinal  cut- 
ting of  fir,  showing  the  circular  glands  on  the 
vessels    which     distinguish    coniferous    woods. 
These  latter  objects  may  be  seen  by  half  -inch  or 
quarter-inch  achromatic  glasses.     Opaque  objects 
are  generally  better  exhibited  by  inch  and  two- 
inch  glasses,  when  a  general  view  of  them  is  re- 
quired, and  by  higher  powers  when  we  wish  to 
examine  their  minute  structure.     In  the  latter 
case  the  light  must  be  obtained  by  condensing 
lenses  instead  of  the  metallic  specula. 
Fig.  28.  Although  the  reflecting    microscope    is    now 

very  little  used,  it  may  be  expected  that  we 
should  mention  it.  In  this  instrument,  at  Fig.  29,  the  object 
O  is  reflected  by  the  inclined  face  of  the  mirror  M,  and  the 
rays  are  again  reflected  and  converged  by  the  ellipsoidal  re- 
flector B  B,  which  eflects  the 
same  purpose  as  the  object-glass 
of  the  compound  microscope. 
It  forms  an  image  which  is  not 
susceptible  of  the  over-correc- 
tion  as  to  color  before  described, 
and  which  therefore  becomes 
colored  in  passing  through  the 
eye-piece.  This  fact,  and  the 

loss  of  light  by  reflection,  will  probably  always  render  the  re- 
flecting microscope  inferior  to  the  achromatic  refracting. 

The  solar  microscope  has  been  so  nearly  superseded  by  the 
oxy-hydrogen,  that  a  brief  description  of  the  latter  must  suf- 
fice, particularly  as  their  optical  principles  are  similar. 
The  primary  object  in  both  is  to  throw  an  intense  light  upon 


Fig.  29. 


THE    MICROSCOPE. 


the  object,  which  is  sometimes  done  by  mirrors,  and  sometimes 
by  lenses.  In  Fig.  30,  L  represents  the  cylinder  of  burning 
lime,  E  E  the  reflector,  which  concentrates  the  light  upon  the 


Fig.  30. 

object  O  O;  the  rays  from  which,  passing  through  the  two  plano- 
convex lenses,  are  brought  to  foci  upon  a  screen  placed  at  a  great 
distance,  and  upon  which  is  formed  the  magnified  image. 

Fig.  31  shows  a  combination  of  lenses  to  condense  the  light 
upon  the  object.  In  either  case  the  optical  arrangements  by 
which  the  image  is  formed  admit  of  the  same  perfection  as 


rATt 


CK 


those  which  have  been  described  for  the  compound  micro- 
scopes. A  few  achromatic  glasses  for  oxy-hydrogen  micro- 
scopes have  been  made,  and  they  will  ultimately  become  valu- 
able instruments  for  illustrating  lectures  on  natural  history  and 
physiology.  One  made  by  Mr.  Eoss  was  exhibited  a  few 
months  since  at  the  Society  of  Arts  to  illustrate  a  lecture  on  the 
physiology  of  woods.  It  should  be  observed,  however,  that 
the  oxy-hydrogen  or  solar  microscope  requires  either  a  spheri- 
cal screen,  or  that  the  objects  should  be  mounted  between 
spherical  glasses,  in  order  to  bring  the  whole  into  focus  at  one 
time.  This  latter  plan  was  adopted  on  the  occasion  just  men- 
tioned with  perfect  success. 


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